Greek character in the reserves (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'the reserves' is the wordplay.
'reserves' becomes 'ta' (I've seen this in other clues).
'the'+'ta'='THETA'
'in' acts as a link.
(Other definitions for theta that I've seen before include "Eighth letter of Greek alphabet" , "letter from abroad" , "character from Greece" , "Foreign letter" , "Foreign character" .)