Tough serviceman allowed in free (5)
I believe the answer is:
rigid
'tough' is the definition.
(thesaurus)
'serviceman allowed in free' is the wordplay.
'serviceman' becomes 'gi' (informal term for a US soldier).
'allowed in' indicates putting letters inside (I've seen this in other clues).
'free' becomes 'rid' (to be free of something is to be rid of it).
'gi' going within 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Incapable of or resistant to bending" , "Firmly fixed" , "strict" , "Stiff, unbending" , "completely" .)