Ultimately fails in plea about trap (5)
I believe the answer is:
snare
'trap' is the definition.
(synonyms)
'ultimately fails in plea about' is the wordplay.
'ultimately' indicates one should take the final letters.
'about' becomes 're' ('re' means regarding or about).
The last letters of 'fails in plea' are 'sna'.
'sna'+'re'='SNARE'
(Other definitions for snare that I've seen before include "Kind of trap, often with noose" , "Noosed trap" , "Wire on drum; trap" , "Trap or entangle" , "Catch" .)