Old stager is a cavalryman? Sounds like it (7)
I believe the answer is:
trouper
'old stager' is the definition.
(I've seen this before)
'a cavalryman? sounds like it' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for trouper that I've seen before include "actor" , "Very reliable performer" , "Old stager" , "reliable worker" , "Experienced entertainer" .)