Blockhead losing pawn, leading to mate (4)
I believe the answer is:
chum
'mate' is the definition.
(thesaurus)
'blockhead losing pawn' is the wordplay.
'blockhead' becomes 'chump' (I've seen this before).
'losing' suggests deleting specific letters.
'pawn' becomes 'p'.
'chump' with 'p' taken out is 'CHUM'.
'leading to' is the link.
(Other definitions for chum that I've seen before include "Pal, mate" , "Pal, buddy" , "Salmon" , "Good friend" , "Close friend (colloq.)" .)