Dowdy swimmer holds backside (8)
I believe the answer is:
frumpish
'dowdy' is the definition.
(synonyms)
'swimmer holds backside' is the wordplay.
I cannot really see how this works, but
'backside' could be 'rump' (synonyms) and 'rump' is found within the answer.
'swimmer' could be 'fish' (fish swim) and 'fish' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for frumpish that I've seen before include "Dowdy" , "unattractive" .)