Correspond with supporter after end of test (5)
I believe the answer is:
tally
'correspond with' is the definition.
(I've seen this before)
'supporter after end of test' is the wordplay.
'supporter' becomes 'ally' (I've seen this before**).
'after' means one lot of letters go next to another.
'end of' suggests the final letters.
The last letter of 'test' is 't'.
'ally' put after 't' is 'TALLY'.
(Other definitions for tally that I've seen before include "Agree - count" , "Mark" , "Coincide" , "Record; current score" , "Square" .)