Wounded GI and the unknown pensioner might be so old (6)
I believe the answer is:
eighty
'pensioner might be so old' is the definition.
(an old-age pensioner could be 80)
'wounded gi and the unknown' is the wordplay.
'wounded' indicates an anagram (I've seen this in other clues).
'and' means one lot of letters go next to another (I've seen this in other clues).
'unknown' becomes 'y' (algebra).
'gi' put after 'the' is 'thegi'.
'thegi' anagrammed gives 'eight'.
'eight'+'y'='EIGHTY'
(Other definitions for eighty that I've seen before include "Fourscore" , "Several score" , "score less than a hundred" , "XXC" , "22 [FOUR] scores" .)