Substitute, first of defenders booked (8)
I believe the answer is:
reserved
'booked' is the definition.
('reserve' can be a synonym of 'book')
'substitute first of defenders' is the wordplay.
'substitute' becomes 'reserve' ('reserve' can be a synonym of 'substitute').
'first of' suggests taking the first letters.
The first letter of 'defenders' is 'd'.
'reserve'+'d'='RESERVED'
(Other definitions for reserved that I've seen before include "Quiet, not given to showing emotion" , "Spoken for" , "Aloof" , "Booked in advance, being somewhat taciturn?" , "Retiring" .)