Hesitate to change key, at first (6)
I believe the answer is:
falter
'hesitate' is the definition.
(faltering is a kind of hesitating)
'change key at first' is the wordplay.
'change' becomes 'alter' ('alter' can be a synonym of 'change'**).
'key' becomes 'f' (I've seen this in other clues).
'at first' says to put letters next to each other.
'alter' put after 'f' is 'FALTER'.
'to' acts as a link.
(Other definitions for falter that I've seen before include "Hesitate, waver" , "Lose momentum" , "Hesitate, begin to give way" , "Hesitate, vacillate" , "Be hesitant" .)