A second C to terminate scale (6)
I believe the answer is:
ascend
'scale' is the definition.
(thesaurus)
'a second c to terminate' is the wordplay.
'a second' becomes 'as' (I've seen this in other clues).
'to terminate' becomes 'end' ('end' can be a synonym of 'terminate').
'as'+'c'+'end'='ASCEND'
(Other definitions for ascend that I've seen before include "Come up" , "Climb, go up (6)" , "Gain altitude" , "Scale" , "Dances (anag.)" .)