Black Sea peninsula loses area in unlawful act (5)
I believe the answer is:
crime
'unlawful act' is the definition.
(I've seen this before)
'black sea peninsula loses area' is the wordplay.
'black sea peninsula' becomes 'crimea' (I've seen this before).
'loses' indicates named letters should be taken away.
'area' becomes 'a' (maths abbreviation).
'crimea' with 'a' taken away is 'CRIME'.
'in' acts as a link.
(Other definitions for crime that I've seen before include "An act punishable by law" , "wrong-doing" , "Illegal activity" , "Offence punishable by law" , "Felony" .)