Thin, a group's leader seen in lake (6)
I believe the answer is:
meagre
'thin' is the definition.
(thesaurus)
'a group's leader seen in lake' is the wordplay.
'leader' suggests taking the first letters.
'seen in' means one lot of letters goes inside another.
'lake' becomes 'mere' (mere is a kind of lake).
The initial letter of 'group' is 'g'.
'a'+'g'='ag'
'ag' placed into 'mere' is 'MEAGRE'.
(Other definitions for meagre that I've seen before include "Inadequate, scanty" , "unsatisfactorily small in amount" , "Skimpy and scarce" , "Lean, thin" , "Small, insufficient" .)