Time of festival — Rev's looking silly in tricorn maybe (7)
I believe the answer is:
harvest
'time of festival' is the definition.
(harvest festival)
'rev's looking silly in tricorn maybe' is the wordplay.
'looking silly' indicates anagramming the letters (a silly spelling).
'in' is an insertion indicator.
'tricorn maybe' becomes 'hat' (tricorn is a type of hat).
'revs' with letters rearranged gives 'rves'.
'rves' placed within 'hat' is 'HARVEST'.
(Other definitions for harvest that I've seen before include "Cut down" , "The fruits of tillage" , "Bring in the corn etc" , "Crop-gathering time" , "Yield from plants in a single growing season" .)