Deserter on time was first to be disconcerted (7)
I believe the answer is:
rattled
'be disconcerted' is the definition.
(I've seen this before)
'deserter on time was first' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'was first' becomes 'led' (to lead is to go first).
'rat'+'t'+'led'='RATTLED'
'to' is the link.
(Other definitions for rattled that I've seen before include "Upset" , "made use of 8 [NOISEMAKER]" , "What some snakes did" , "Annoyed; shook" , "evidence of having screw loose?" .)