Loud cry of a colour after dropping OW! (4)
I believe the answer is:
yell
'loud cry' is the definition.
(I've seen this before)
'colour after dropping ow' is the wordplay.
'colour' becomes 'yellow' (yellow is a kind of colour).
'after dropping' is a deletion indicator.
'yellow' with 'ow' taken away is 'YELL'.
'of a' acts as a link.
(Other definitions for yell that I've seen before include "Shout loudly" , "Clement --, politician" , "Shetland isle" , "Loud sharp cry" , "Loud utterance" .)