Letting in one single goal, including defender's header (9)
I believe the answer is:
admission
'letting in' is the definition.
(admit can mean to let in)
'one single goal including defender's header' is the wordplay.
'one single' becomes 'a' (a single example of something).
'goal' becomes 'mission'.
'including' indicates putting letters inside.
'header' indicates taking the first letters (in a down clue, the letter at the top of the word).
The first letter of 'defenders' is 'd'.
'a'+'mission'='amission'
'amission' going around 'd' is 'ADMISSION'.
(Other definitions for admission that I've seen before include "for which one may be charged" , "Entrance fee" , "guilty plea, perhaps" , "charge for entrance" , "Disclosure" .)