Be ardent then change employment (7)
I believe the answer is:
enthuse
'be ardent' is the definition.
I can't tell whether this definition defines the answer.
'then change employment' is the wordplay.
'change' indicates anagramming the letters.
'employment' becomes 'use' (synonyms).
'then' with letters rearranged gives 'enth'.
'enth'+'use'='ENTHUSE'
(Other definitions for enthuse that I've seen before include "Tunes he adapted to feel eager liking for" , "Show lively interest and keenness" , "Seen hut? (anag) - show a lively interest" , "Show great interest" , "Show passionate zeal (about something)" .)