Uncompromising GI getting in free (5)
I believe the answer is:
rigid
'uncompromising' is the definition.
(I've seen this before)
'gi getting in free' is the wordplay.
'getting in' means one lot of letters goes inside another.
'free' becomes 'rid' (synonyms).
'gi' put within 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Stiff and inflexible" , "Frozen" , "Not flexible" , "stern" , "Unable to bend" .)