Struggling to detain a counter-agent (8)
I believe the answer is:
antidote
'a counter-agent' is the definition.
(counter-agent for a poison)
'struggling to detain' is the wordplay.
'struggling' is an anagram indicator (I've seen this in other clues).
'to'+'detain'='todetain'
'todetain' anagrammed gives 'ANTIDOTE'.
(Other definitions for antidote that I've seen before include "Counter-measure" , "It is used to counteract poison" , "Donate it to anti-poison agent" , "It makes you feel better" , "It counteracts poison" .)