Briefly feeling low (3)
I believe the answer is:
moo
'low' is the definition.
(I've seen this before)
'briefly feeling' is the wordplay.
'briefly' means to remove the last letter.
'feeling' becomes 'mood' (mood is a kind of feeling).
'mood' with its last letter taken off is 'MOO'.
(Other definitions for moo that I've seen before include "Cow's call" , "Bovine low" , "Irritating woman" , "unpopular woman" , "Guernsey noise?" .)