Curse the first to appear in sheep's clothing (5)
I believe the answer is:
swear
'curse the first' is the definition.
'swear' can be an answer for 'curse' (synonyms). I am not sure about the 'the first' bit.
'sheep's clothing' is the wordplay.
'sheep's' becomes 's' (this might be a standard abbreviation which I don't know about).
'clothing' becomes 'wear' (synonyms).
's'+'wear'='SWEAR'
'to appear in' acts as a link.
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Utter obscenities or take an oath" , "Vow; blaspheme" , "Avow" , "Promise on oath" , "Emit oath or take vow" .)