Fool's mate? Pressure (5)
I believe the answer is:
chump
'fool's' is the definition.
(synonyms)
'mate? pressure' is the wordplay.
'mate?' becomes 'chum' (I've seen this before).
'pressure' becomes 'p' (abbreviation).
'chum'+'p'='CHUMP'
(Other definitions for chump that I've seen before include "Fool - sort of chop" , "Nitwit" , "Foolish person" , "Blockhead" , "Mug, patsy, sucker" .)