Opposed to conservationists blocking artificial insemination (4)
I believe the answer is:
anti
'opposed to' is the definition.
(I've seen this before)
'conservationists blocking artificial insemination' is the wordplay.
'conservationists' becomes 'NT' (abbreviation for National Trust).
'blocking' indicates putting letters inside.
'artificial insemination' becomes 'AI' (abbreviation for artificial insemination).
'nt' inserted into 'ai' is 'ANTI'.
(Other definitions for anti that I've seen before include "'No' voter" , "not favoured" , "Against, informally" , "Policy's opponent" , "Hostile" .)