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 present in the answer.
'swimmer' could be 'fish' (fish swim) and 'fish' is found within the remaining letters.
No letters remain.
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" .)