It involves mate getting companion to accompany letter (5)
I believe the answer is:
chess
'it involves mate' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'mate' relates to this answer.
'companion to accompany letter' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'to accompany' means one lot of letters go next to another.
'letter' becomes 'ess' (the letter S).
'ch'+'ess'='CHESS'
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for chess that I've seen before include "Benny Andersson and Bjorn Ulvaeus musical" , "Ancient war game whose name may have derived for the Persian for 'king'" , "that has over thirty men on board" , "board meeting" , "Sixteen-men-a-side game" .)