Stag full of energy and courage (5)
I believe the answer is:
heart
'courage' is the definition.
(heart is a kind of courage)
'stag full of energy' is the wordplay.
'stag' becomes 'hart' ('hart' can be a synonym of 'stag').
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'hart' enclosing 'e' is 'HEART'.
'and' is the link.
(Other definitions for heart that I've seen before include "Cardiac courage" , "Decorated (cake)" , "Spirit - core" , "bottle" , "Playing card - pity" .)