Firm retaining trainee initially showing little interest (6)
I believe the answer is:
stolid
'showing little interest' is the definition.
(I've seen this before)
'firm retaining trainee initially' is the wordplay.
'firm' becomes 'solid' (synonyms).
'retaining' means one lot of letters goes inside another (inserted letters are held or retained).
'initially' says to take the initial letters.
The first letter of 'trainee' is 't'.
'solid' enclosing 't' is 'STOLID'.
(Other definitions for stolid that I've seen before include "Calm, unanimated" , "Impassive, unemotional" , "Hard to excite" , "Unemotional, unresponsive" , "Not excitable, doesn't show emotion" .)