Hunk starts to stretch lats and biceps (4)
I believe the answer is:
slab
'hunk' is the definition.
(I've seen this before)
'starts to stretch lats and biceps' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'stretch lats and biceps' is 'SLAB'.
(Other definitions for slab that I've seen before include "Piece of paving" , "Paving stone" , "Flat mass of stone" , "Hunk" , "Flat broad piece as of stone" .)