Pressure to spearhead appeal (5)
I believe the answer is:
plead
'appeal' is the definition.
(pleading is a kind of appealing)
'pressure to spearhead' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'to spearhead' becomes 'lead' (I've seen this before).
'p'+'lead'='PLEAD'
(Other definitions for plead that I've seen before include "Implore, entreat" , "Say one is guilty or not guilty" , "Present an argument in court" , "Request" , "Claim; entreat" .)