Son covered in blubber is to cut back on food (4)
I believe the answer is:
fast
'to cut back on food' is the definition.
(to fast is to refrain from eating food)
'son covered in blubber' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'covered in' is an insertion indicator.
'blubber' becomes 'fat' (similar in meaning).
's' going inside 'fat' is 'FAST'.
'is' is the link.
(Other definitions for fast that I've seen before include "Rapid; abstain from food" , "Period during which fare is reduced" , "Don't scoff" , "Maybe bound to spot" , "Renounce food; quick" .)