Employing thinking, but not initially (5)
I believe the answer is:
using
'employing' is the definition.
(to employ is to use)
'thinking but not initially' is the wordplay.
'thinking' becomes 'musing' (muse can mean to think about something).
'but not initially' means to remove the first letter.
'musing' with its first letter taken off is 'USING'.
(Other definitions for using that I've seen before include "Employing" , "practice" , "addiction" , "having recourse to" , "Exploiting for one's own advantage" .)