Porter confused about old soldier (7)
I believe the answer is:
trooper
'soldier' is the definition.
(I know that trooper is a type of soldier)
'porter confused about old' is the wordplay.
'confused' is an anagram indicator.
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'porter' anagrammed gives 'troper'.
'troper' placed around 'o' is 'TROOPER'.
(Other definitions for trooper that I've seen before include "Private soldier" , "Cavalryman" , "ship" , "Cavalry soldier" , "Proverbially swearing soldier" .)