One in lake with article that's flexible (5)
I believe the answer is:
lithe
'flexible' is the definition.
(I've seen this before)
'one in lake with article' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'with' is a charade indicator (letters next to each other).
'article' becomes 'the' (the definite article).
'l'+'the'='lthe'
'i' going inside 'lthe' is 'LITHE'.
'that's' is the link.
(Other definitions for lithe that I've seen before include "Slim and supple" , "Physically flexible or supple" , "Lissom, supple" , "quick to move" , "grace" .)