Grumpy at the empty pen (5)
I believe the answer is:
testy
'grumpy' is the definition.
(testy can mean irritable or annoyed)
'at the empty pen' is the wordplay.
'at' is a charade indicator (letters next to each other).
'empty' means to remove the middle letters.
'pen' becomes 'sty' (both can mean an animal enclosure).
'the' with its centre taken out is 'te'.
'te' next to 'sty' is 'TESTY'.
(Other definitions for testy that I've seen before include "Ratty" , "Easily annoyed" , "Irascible" , "Easily irritated" , "Irritable" .)