Old rebel leader, terribly disheartened, left in rain? (3,5)

'old rebel' is the definition.
'wat tyler' can be an answer for 'rebel' (I've seen this before). I am not certain of the 'old' bit.

'leader terribly disheartened left in rain?' is the wordplay.
'leader' says to take the initial letters.
'disheartened' becomes 'ly' (I can't explain this - if you can you should believe this answer much more).
'left' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'rain?' becomes 'water' (rainwater).
The initial letter of 'terribly' is 't'.
'ly' backwards is 'yl'.
't'+'yl'='tyl'
'tyl' inserted into 'water' is 'WAT TYLER'.

Can you help me to learn more?