On replacing a fuse, a turn to the left is beneficial (6)
I believe the answer is:
useful
'beneficial' is the definition.
(I've seen this in another clue)
'on replacing a fuse a turn to the left' is the wordplay.
'on' indicates putting letters inside (as in clothing 'on' a person).
'replacing' is an anagram indicator (placing again).
'a turn' becomes 'U' (a 'U-turn' is a kind of turn).
'to the left' becomes 'L' (common abbreviation).
'fuse' is an anagram of 'sefu'.
'u'+'l'='ul'
'sefu' put within 'ul' is 'USEFUL'.
'is' acts as a link.
(Other definitions for useful that I've seen before include "Proficient; handy" , "Of practical assistance" , "Handy; very able" , "profitable" , "Of service" .)