Digit: it's one occasionally (3)
I believe the answer is:
toe
'digit' is the definition.
(digit on a foot)
'it's one occasionally' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'ts'+'one'='tsone'
The alternating letters of 'tsone' are 'TOE'.
(Other definitions for toe that I've seen before include "'Little piggy'" , "Front of shoe" , "A foot division" , "One with sock on" , "Calabria?" .)