Secretive and occasionally sulky (3)
I believe the answer is:
sly
'secretive' is the definition.
(I've seen this in other clues)
'occasionally sulky' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternate letters of 'sulky' are 'SLY'.
'and' acts as a link.
(Other definitions for sly that I've seen before include "Cunning, deceitful" , "Devious and furtive" , "Doing things in unpleasant and secret way" , "Crafty, devious" , "Unpleasantly cunning and secretive" .)