Lend a hand when it's spoilt by fool (6)
I believe the answer is:
assist
'lend a hand' is the definition.
(I've seen this in another clue)
'it's spoilt by fool' is the wordplay.
'spoilt' is an anagram indicator.
'by' means one lot of letters go next to another.
'fool' becomes 'ass' (both can mean a stupid person).
'its' with letters rearranged gives 'ist'.
'ist' put after 'ass' is 'ASSIST'.
'when' acts as a link.
(Other definitions for assist that I've seen before include "Help member" , "Serve" , "provide backing" , "Understand by instinct" , "Pass to scorer" .)