Dog's firm resistance when going over grit occasionally (5)
I believe the answer is:
corgi
'dog's' is the definition.
(corgi is a kind of dog)
'firm resistance when going over grit occasionally' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'resistance when' becomes 'r' (I've seen this in other clues).
'going over' is a charade indicator (letters next to each other).
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternating letters of 'grit' are 'gi'.
'co'+'r'+'gi'='CORGI'
(Other definitions for corgi that I've seen before include "Royal canine" , "Welsh breen of dog" , "Pre-2018 royal dog" , "Dog of Welsh breed, royal favourite" , "Welsh breed of short-legged dog" .)