Queen maybe inspires a rare measure in Hatton Garden (5)
I believe the answer is:
'hatton garden' is the definition.
I can't tell whether this defines the answer.
'queen maybe inspires a rare measure' is the wordplay.
'queen maybe' becomes 'cat' (a female cat is a queen).
'inspires' means one lot of letters goes inside another (inspire means to breathe in).
'a rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'measure' becomes 'a' (A is an example).
'cat' enclosing 'ra' is 'CARAT'.
'in' acts as a link.
(Other definitions for carat that I've seen before include "Measure of weight of diamonds" , "Unit of weight for diamonds" , "Gemstone weight unit" , "Measure for diamonds or gold" , "Unit of weight of gemstones" .)