Terrible job holding an instrument (5)
I believe the answer is:
banjo
'instrument' is the definition.
(banjo is a kind of instrument)
'terrible job holding an' is the wordplay.
'terrible' indicates an anagram.
'holding' means one lot of letters goes inside another.
'job' is an anagram of 'bjo'.
'bjo' going around 'an' is 'BANJO'.
(Other definitions for banjo that I've seen before include "No jab (anag.)" , "strumming instrument" , "Stringed instrument with long neck" , "String instrument" , "Instrument with long neck and round body" .)