An element of compassion embodies maturity essentially (7)
I believe the answer is:
mercury
'an element' is the definition.
(mercury is a chemical element)
'compassion embodies maturity essentially' is the wordplay.
'compassion' becomes 'mercy' (mercy is a kind of compassion).
'embodies' indicates putting letters inside.
'essentially' says to take the centre.
The central letters of 'maturity' are 'ur'.
'mercy' enclosing 'ur' is 'MERCURY'.
'of' is the link.
(Other definitions for mercury that I've seen before include "Element, symbol Hg" , "Planet - liquid metal" , "First planet from the Sun" , "Planet next to the sun" , "Atomic number 80" .)