Mandy replaced ring on generator (6)
I believe the answer is:
dynamo
'generator' is the definition.
(dynamo is a kind of generator)
'mandy replaced ring on' is the wordplay.
'replaced' indicates an anagram.
'ring' becomes 'o'.
'on' says to put letters next to each other.
'mandy' is an anagram of 'dynam'.
'dynam'+'o'='DYNAMO'
(Other definitions for dynamo that I've seen before include "One creating electricity" , "Power source" , "Live wire" , "Electric current generator" , "Juice supplier" .)