Figure obtained by doing diet occasionally (5)
I believe the answer is:
digit
'figure' is the definition.
(numerical figure)
'doing diet occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'doing'+'diet'='doingdiet'
The alternating letters of 'doingdiet' are 'DIGIT'.
'obtained by' acts as a link.
(Other definitions for digit that I've seen before include "Need a bit of a hand" , "One's on hand" , "Finger, thumb or toe" , "one of several at hand" , "E.g. toe" .)