After exercise bubblier, regularly lost a stone (6)
I believe the answer is:
pebble
'a stone' is the definition.
(pebble is a kind of stone)
'after exercise bubblier regularly lost' is the wordplay.
'after' means one lot of letters go next to another.
'exercise' becomes 'pe' (Physical Education).
'regularly lost' indicates alternate letters (letters removed at regular intervals).
The alternate letters of 'bubblier' are 'bble'.
'pe' next to 'bble' is 'PEBBLE'.
(Other definitions for pebble that I've seen before include "Stone on beach" , "Small, round stone" , "Small fragment of rock" , "There are many on a stony beach" , "Little rock" .)