Lake of same regularity (4)
I believe the answer is:
mere
'lake' is the definition.
(I know that mere is a type of pond)
'of same regularity' is the wordplay.
'of' indicates a hidden word.
'MERE' can be found hidden inside 'same regularity'.
(Other definitions for mere that I've seen before include "a drop of water" , "Paltry" , "Lake (poetic); Maori war club" , "Nothing more than a small pond" , "Pool, small lake" .)