Delicate force in ebbing river (5)
I believe the answer is:
elfin
'delicate' is the definition.
(I've seen this before)
'force in ebbing river' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'in' indicates putting letters inside.
'ebbing' says the letters should be written in reverse (to ebb can mean to go backwards).
'river' becomes 'nile' (Nile is an example**).
'nile' reversed gives 'elin'.
'f' placed inside 'elin' is 'ELFIN'.
(Other definitions for elfin that I've seen before include "Small, delicately-framed" , "Petite and charming" , "Dainty" , "Delicate little" , "Fairy-like" .)