Tight son refuses to eat (5)
I believe the answer is:
fasts
'refuses to eat' is the definition.
'tight son' is the wordplay.
'tight' becomes 'fast' (both can mean secure).
'son' becomes 's'.
'fast'+'s'='FASTS'
(Other definitions for fasts that I've seen before include "Times of abstinence" , "Abstains from food" , "Goes without food" , "Goes hungry" , "Avoids food" .)