Agile male beginning to bathe in river (6)
I believe the answer is:
nimble
'agile' is the definition.
(synonyms)
'male beginning to bathe in river' is the wordplay.
'male' becomes 'm' (common abbreviation).
'beginning to' says to take the initial letters.
'in' indicates putting letters inside.
'river' becomes 'nile' (Nile River is an example).
The first letter of 'bathe' is 'b'.
'm'+'b'='mb'
'mb' inserted inside 'nile' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Quick on the feet" , "Agile - alert" , "Swift, active" , "'Jack be ......, Jack be quick' (6)." , "Moving quickly and lightly" .)