Header missed in game after trial for Roman defence (7)
I believe the answer is:
testudo
'roman defence' is the definition.
(defensive structure used by Romans)
'header missed in game after trial' is the wordplay.
'header missed in' means to remove the first letter.
'game' becomes 'ludo' (ludo is a board game).
'after' says to put letters next to each other.
'trial' becomes 'test' (synonyms).
'ludo' with its first letter removed is 'udo'.
'udo' after 'test' is 'TESTUDO'.
'for' acts as a link.
(Other definitions for testudo that I've seen before include "cover for safety" , "Roman defence with shields" , "Roman air-raid shelter?" , "cover for troops" , "Battle formation using shields as umbrellas" .)