Collectively backs excuse (7)
I believe the answer is:
defence
This clue might be a double definition.
'collectively backs' is the first definition.
'defence' can be an answer for 'backs' (the backs make up the defence in a football team). I'm not certain of the 'collectively' bit.
'excuse' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for defence that I've seen before include "barricade" , "Armed forces' role" , "team of lawyers" , "The D in MoD" , "Justification for some act" .)