Killer whale lying back by sea, not large, certainly agile (9)
I believe the answer is:
acrobatic
'agile' is the definition.
(I've seen this before)
'killer whale lying back by sea not large certainly' is the wordplay.
I cannot really understand how this works, but
'by' could be 'at' and 'at' is present in the answer.
This explanation may well be incorrect...
(Other definitions for acrobatic that I've seen before include "Like a tumbling or swinging performer" , "spectacularly agile" , "circus performer?" , "Involving physical agility" , "Performing spectacular gymnastic feats" .)