Unit employed with rocks to lug round area (5)
I believe the answer is:
carat
'unit employed with rocks' is the definition.
(unit of weight for precious stones)
'lug round area' is the wordplay.
'lug' becomes 'cart' (both can mean to carry or transport).
'round' is an insertion indicator.
'area' becomes 'a' (maths abbreviation).
'cart' going around 'a' is 'CARAT'.
'to' is the link.
(Other definitions for carat that I've seen before include "A unit of weight for precious stones" , "Weight of ice" , "Jewellery unit" , "Not a large weight" , "Measure of purity of gold" .)