Looks up abbreviation to find it denotes action (4)
I believe the answer is:
verb
'it denotes action' is the definition.
The definition suggests a singular noun which matches the answer.
'looks up abbreviation to find' is the wordplay.
'looks up' says the letters should be written in reverse (I've seen 'looking up' mean this (in a down clue, letters go up)).
'to find' indicates a hidden word (I've seen 'found' mean this).
'brev' is hidden in the letters of 'abbreviation'.
'brev' written backwards gives 'VERB'.
Can you help me to learn more?
(Other definitions for verb that I've seen before include "Predicate of a sentance (4)" , "Part of speech" , "Type of word indicating action" , "Maybe protest" , "Word that denotes an action or a state" .)