Mount of a leader of cavalry wearing mail (6)
I believe the answer is:
ascend
'mount' is the definition.
(I've seen this before)
'a leader of cavalry wearing mail' is the wordplay.
'leader of' says to take the initial letters.
'wearing' means one lot of letters goes inside another.
'mail' becomes 'send' (synonyms).
The first letter of 'cavalry' is 'c'.
'c' going into 'send' is 'scend'.
'a'+'scend'='ASCEND'
'of' acts as a link.
(Other definitions for ascend that I've seen before include "Gain height" , "Mount" , "Make the top" , "Climb, rise" , "Move upward" .)