Strait-laced soldier getting in free (5)
I believe the answer is:
rigid
'strait-laced' is the definition.
(I've seen this in another clue)
'soldier getting in free' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'getting in' is an insertion indicator.
'free' becomes 'rid' (similar in meaning).
'gi' going within 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Stiff and inflexible" , "not relaxing" , "Unable to bend" , "Firmly fixed" , "Frozen" .)