Fit in curve round lake (5)
I believe the answer is:
blend
'fit in' is the definition.
(blending is a kind of fitting in)
'curve round lake' is the wordplay.
'curve' becomes 'bend' (bend is a kind of curve).
'round' means one lot of letters goes inside another.
'lake' becomes 'l'.
'bend' placed around 'l' is 'BLEND'.
(Other definitions for blend that I've seen before include "Mix together" , "Combine together" , "Mingle" , "Amalgam" , "Mix and combine" .)