Shout cowardly, letting out expression of pain (4)
I believe the answer is:
yell
'shout' is the definition.
(yelling is a kind of shouting)
'cowardly letting out expression of pain' is the wordplay.
'cowardly' becomes 'yellow' ('yellow' can be similar in meaning to 'cowardly').
'letting out' suggests deleting specific letters.
'expression of pain' becomes 'ow'.
'yellow' with 'ow' removed is 'YELL'.
(Other definitions for yell that I've seen before include "Scream" , "Howl" , "Loud sharp cry" , "Shout - Shetland island" , "Clement --, politician" .)