This soldier could be one GI in real trouble (11)
I believe the answer is:
legionnaire
'this soldier' is the definition.
(legionnaire is a kind of soldier)
'one gi in real trouble' is the wordplay.
'trouble' indicates anagramming the letters.
'one'+'gi'+'in'+'real'='onegiinreal'
'onegiinreal' is an anagram of 'LEGIONNAIRE'.
'could be' is the link.
(Other definitions for legionnaire that I've seen before include "Beau Geste was one" , "French soldier" .)