sorry, but i don't think you'll find a neat notional definition. there are clues for distinguishing them though.
their intonation, for one. in 'a prosecutor in boston' the stress
is on 'prosecutor', the head. in 'prisoner of war' the primary
stress is on 'war'.
if you take 'prisoner of war' as a lexeme - as you say, roughly
corresponding to headwords of dictionaries - is it syntactically
opaque? can you insert other constructions into it like you can
'a prosecutor in boston':
(1) There is a prosecutor called Sam in Boston.
(2) *There is a prisoner called Sam of war.
you can rephrase (1) to have a relative clause instead of the PP, but can you do the same with (2)?
how about
one-substitution?
(3) There is a prosecutor in New York and one in Boston.
(4) ?He is a prisoner of war and one of conscience.
'one' in (3) refers to 'prosecutor', not 'prosecutor in New
York'. in (4) it refers to 'prisoner of war', not just
'prisoner', so the collocation 'prisoner of conscience' does not occur.
food for thought.
sam