Amazing soldier enters river doing a backflip (5)
I believe the answer is:
magic
'amazing' is the definition.
'soldier enters river doing a backflip' is the wordplay.
'soldier' becomes 'gi' (GI can mean a US soldier).
'enters' indicates putting letters inside.
'river' becomes 'cam' (river in Cambridge, England).
'doing a backflip' is a reversal indicator.
'cam' in reverse letter order is 'mac'.
'gi' inserted within 'mac' is 'MAGIC'.
(Other definitions for magic that I've seen before include "Unexpectedly produce" , "Witchcraft" , "Fungus causing trips?" , "Wizardry - wizard!" , "Spelling?" .)