Grumpy sort of communist that you must punch (5)
I believe the answer is:
trout
'grumpy sort' is the definition.
(derogatory term for an unpleasant person)
'communist that you must punch' is the wordplay.
'communist' becomes 'trot' (short for Trotskyist).
'that you' becomes 'u' (I've seen this in other clues).
'must punch' indicates putting letters inside (can mean 'make a hole in', as in 'hole-punch').
'trot' placed around 'u' is 'TROUT'.
'of' is the link.
(Other definitions for trout that I've seen before include "Swimmer" , "Irritating old person" , "It rises to fly" , "tickled to death?" , "Game fish related to salmon" .)