Copper dependable? Not at first (5)
I believe the answer is:
rusty
'copper' is the definition.
(thesaurus)
'dependable? not at first' is the wordplay.
'dependable?' becomes 'trusty' ('trusty' can be similar in meaning to 'dependable').
'not at first' means to remove the first letter.
'trusty' with its first letter taken away is 'RUSTY'.
(Other definitions for rusty that I've seen before include "affected by inactivity" , "Reddish," , "(Of iron) corroded" , "Antiquated" , "showing sign of inactivity?" .)