Corporation screened by agent in squat (6)
I believe the answer is:
stumpy
'squat' is the definition.
(I know that squat can be written as stumpy)
'corporation screened by agent' is the wordplay.
'corporation' becomes 'tum' (I've seen this before).
'screened by' is an insertion indicator.
'agent' becomes 'spy' (spy is a kind of agent).
'tum' placed into 'spy' is 'STUMPY'.
'in' acts as a link.
(Other definitions for stumpy that I've seen before include "pudgy" , "Short and thick" , "Short and thickset" , "Short and squat" .)