A number score twice with only headers (6-3)
I believe the answer is:
twenty-two
'a number' is the definition.
(twenty-two is a kind of whole number)
'score twice with only headers' is the wordplay.
'score' becomes 'twenty' (synonyms).
'headers' indicates taking the first letters (I've seen 'header' mean this (in a down clue, the letter at the top of the word)).
The first letters of 'twice with only' is 'two'.
'twenty'+'two'='TWENTY-TWO'
(Other definitions for twenty-two that I've seen before include "Number of players on pitch at start of soccer match" , "Number of yards in a chain" , "Two football teams" , "Line on rugby pitch; number" , "One more than this clue number" .)