Thin lake with a bit of greenery within (6)
I believe the answer is:
meagre
'thin' is the definition.
(I've seen this before)
'lake with a bit of greenery within' is the wordplay.
'lake' becomes 'mere' (mere is a kind of lake).
'with' is an insertion indicator.
'bit of' suggests taking the first letters.
'within' is an insertion indicator.
The first letter of 'greenery' is 'g'.
'mere' placed around 'a' is 'meare'.
'meare' placed around 'g' is 'MEAGRE'.
(Other definitions for meagre that I've seen before include "Scant" , "Lacking in quantity or quality" , "unsatisfactorily small in amount" , "weak" , "Sparse" .)