Sort of duck, fast mover, one of five crossing lake (9)
I believe the answer is:
harlequin
'sort of duck' is the definition.
(type of duck)
'fast mover one of five crossing lake' is the wordplay.
'fast mover' becomes 'hare' (hares run fast).
'one of five' becomes 'quin' (I've seen this before).
'crossing' indicates putting letters inside.
'lake' becomes 'l' (geographical abbreviation).
'hare'+'quin'='harequin'
'harequin' enclosing 'l' is 'HARLEQUIN'.
(Other definitions for harlequin that I've seen before include "Pantomime character with Columbine - rugby player" , "Northern duck" , "Traditional pantomime character" , "Entertainer in diamond-patterned clothing" , "Character in a diamond-patterned costume - variegated" .)