Level of aid mainly accepted by European prisoner (7)
I believe the answer is:
echelon
'level' is the definition.
(I've seen this before)
'aid mainly accepted by european prisoner' is the wordplay.
'aid' becomes 'help' ('help' can be a synonym of 'aid').
'mainly' means to remove the last letter.
'accepted by' means one lot of letters goes inside another.
'european' becomes 'e' (abbreviation e.g. EU).
'prisoner' becomes 'con' (abbreviation for 'convict').
'help' with its final letter removed is 'hel'.
'e'+'con'='econ'
'hel' inserted inside 'econ' is 'ECHELON'.
'of' acts as a link.
(Other definitions for echelon that I've seen before include "Body of troops arranged in a line" , "Echo Len (anag.)" , "Level or rank in a group" , "Ladder formation" , "Leech on (anag.)" .)