Tool — one wasn't working (5,3)
I believe the answer is:
tenon saw
'tool' is the definition.
(I've seen this before)
'one wasn't working' is the wordplay.
'working' indicates anagramming the letters.
'one'+'wasnt'='onewasnt'
'onewasnt' is an anagram of 'TENON-SAW'.
(Other definitions for tenon saw that I've seen before include "Carpenter's tool" , "used in joint production" , "One cuts" , "Precision cutter for making wooden joints" , "Woodcutter" .)