Grumpy at the empty pen (5)
I believe the answer is:
'grumpy' is the definition.
(I've seen this before)
'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' (sty is a kind of pen).
'the' with its middle removed is 'te'.
'te' next to 'sty' is 'TESTY'.
(Other definitions for testy that I've seen before include "Irascible", "Ratty", "Irritable", "likely to flare up?", "Cross".)