Serpentine's green, and I'm swimming (10)
I believe the answer is:
meandering
'serpentine's' is the definition.
(thesaurus)
'green and i'm swimming' is the wordplay.
'swimming' indicates anagramming the letters.
'green'+'and'+'im'='greenandim'
'greenandim' with letters rearranged gives 'MEANDERING'.
(Other definitions for meandering that I've seen before include "convoluted" , "not in straight line" , "full of twists and turns" , "Aimlessly wandering" , "Tortuous" .)