Puckish fairy originally in river doing the backstroke? (5)
I believe the answer is:
elfin
'puckish' is the definition.
(thesaurus)
'fairy originally in river doing the backstroke?' is the wordplay.
'originally' indicates taking the first letters.
'in' indicates putting letters inside.
'river' becomes 'nile' (Nile is an example**).
'doing the backstroke?' says the letters should be written in reverse.
The first letter of 'fairy' is 'f'.
'f' going inside 'nile' is 'nifle'.
'nifle' in reverse letter order is 'ELFIN'.
(Other definitions for elfin that I've seen before include "Small, delicate, mischievous and attractive" , "puckish" , "Like a mischievous fairy" , "Delicately framed" , "Sweet" .)