Though full of steak in extremes of gaiety, quick to complain (6)
I believe the answer is:
grumpy
'quick to complain' is the definition.
I can't judge whether this definition defines the answer.
'though full of steak in extremes of gaiety' is the wordplay.
'though full of steak' becomes 'rump' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'extremes of' says to hollow out the word (remove centre letters).
'gaiety' with its centre taken out is 'gy'.
'rump' put into 'gy' is 'GRUMPY'.
Can you help me to learn more?
(Other definitions for grumpy that I've seen before include "One of Snow White's Famous Seven" , "likely to snap?" , "Bad-tempered, sulky" , "animated little miner" , "short one" .)