Petition putting pressure on chief (5)
I believe the answer is:
plead
'petition' is the definition.
(I've seen this before)
'pressure on chief' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' means one lot of letters go next to another.
'chief' becomes 'lead'.
'p'+'lead'='PLEAD'
'putting' acts as a link.
(Other definitions for plead that I've seen before include "Make an emotional appeal" , "Argue (for a cause)" , "Request" , "Say one is guilty or not guilty" , "Present an argument in court" .)