Had lunch in canteen, occasionally (3)
I believe the answer is:
ate
'had lunch' is the definition.
(I've seen this in another clue)
'canteen occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
The alternate letters of 'canteen' are 'ATE'.
'in' is the link.
(Other definitions for ate that I've seen before include "Took food" , "upset > Greek goddess" , "Bolted" , "Worried" , "Munched" .)