Put out when fun's spoilt by a fathead (5)
I believe the answer is:
snuff
'put out' is the definition.
(I've seen this before)
'fun's spoilt by a fathead' is the wordplay.
'spoilt' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'a fathead' becomes 'f' (head letter of 'fat').
'funs' anagrammed gives 'snuf'.
'snuf'+'f'='SNUFF'
'when' is the link.
(Other definitions for snuff that I've seen before include "Trim (candle); some tobacco" , "Powdered tobacco product" , "Powdered tobacco taken through the nose" , "powder?" , "Extinguish - powdered tobacco" .)