Kit Harington's first shirt fitting (5)
I believe the answer is:
right
'fitting' is the definition.
(I've seen this before)
'kit harington's first shirt' is the wordplay.
'kit' becomes 'rig' (I've seen this before).
'harington's first' becomes 'h' (1st letter of 'harington').
'shirt' becomes 't' (as in a T-shirt).
'rig'+'h'+'t'='RIGHT'
(Other definitions for right that I've seen before include "licence" , "Valid" , "''The customer is always .....''" , "One has a claim to be correct" , "Morally good; rectify" .)