Exercise gets first of bulky blokes losing eight stone (6)
I believe the answer is:
pebble
'stone' is the definition.
(pebble is a kind of stone)
'exercise gets first of bulky blokes losing eight' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'gets' says to put letters next to each other.
'first of' says to take the initial letters.
The first letters of 'bulky blokes losing eight' is 'bble'.
'pe'+'bble'='PEBBLE'
(Other definitions for pebble that I've seen before include "Smooth little stone" , "A small smooth stone" , "There are many on a stony beach" , "Small, round stone" , "Small fragment of rock" .)