Craftsman, if erratic, breaks down (9)
I believe the answer is:
artificer
'craftsman' is the definition.
(I have seen 'Skilled craftsman' mean 'artificer' so perhaps 'craftsman' could also mean 'artificer')
'if erratic breaks down' is the wordplay.
'breaks down' indicates anagramming the letters.
'if'+'erratic'='iferratic'
'iferratic' anagrammed gives 'ARTIFICER'.
(Other definitions for artificer that I've seen before include "Inventor who's skilful" , "Military mechanic" , "Skilled craftsman" , "Mechanic in the armed forces" , "One into skill" .)