Black bird round a lake to see swell (7)
I believe the answer is:
balloon
'swell' is the definition.
(both can mean to expand or increase)
'black bird round a lake' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'bird' becomes 'loon' (loon is a kind of bird).
'round' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'b'+'loon'='bloon'
'a'+'l'='al'
'bloon' enclosing 'al' is 'BALLOON'.
'to see' is the link.
(Other definitions for balloon that I've seen before include "See 21" , "Inflatable - rapidly expand" , "plaything" , "In which Charlie Brown expressed himself" , "Inflate" .)