Old soldier sure about crossing river (5)
I believe the answer is:
sepoy
'old soldier' is the definition.
'sepoy' can be an answer for 'soldier' (I've seen this before). I am not certain of the 'old' bit.
'sure about crossing river' is the wordplay.
'sure' becomes 'yes' (both are affirmative answers).
'about' says the letters should be written backwards.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'Po' (Italian river).
'yes' in reverse letter order is 'sey'.
'sey' going around 'po' is 'SEPOY'.
Can you help me to learn more?
(Other definitions for sepoy that I've seen before include "(Former) Indian soldier in the British service" , "Eastern soldier" , "Native soldier" , "Indian soldier serving under British rule" , "Raj cop" .)