Thrifty fellow covering a lake (6)
I believe the answer is:
frugal
'thrifty' is the definition.
('frugal' can be similar in meaning to 'thrifty')
'fellow covering a lake' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'covering' becomes 'rug' (rug is a kind of covering**).
'lake' becomes 'l'.
'f'+'rug'+'a'+'l'='FRUGAL'
(Other definitions for frugal that I've seen before include "Economical with food or money" , "Taking care" , "Thrifty - meagre" , "Practising economy" , "Lag fur to be thrifty" .)