OK swimmer keeping in trim, doing the backstroke? (6)
I believe the answer is:
ratify
'ok' is the definition.
(thesaurus)
'swimmer keeping in trim doing the backstroke?' is the wordplay.
'swimmer' becomes 'ray' (ray is a kind of aquatic vertebrate).
'keeping' indicates putting letters inside.
'in trim' becomes 'fit' (I've seen this before).
'doing the backstroke?' is a reversal indicator.
'fit' reversed gives 'tif'.
'ray' going around 'tif' is 'RATIFY'.
(Other definitions for ratify that I've seen before include "Endorse" , "Approve and sign" , "Authenticate" , "Approve officially" , "OK" .)