Demanding pace needs an injection of energy (5)
I believe the answer is:
steep
'demanding' is the definition.
(thesaurus)
'pace needs an injection of energy' is the wordplay.
'pace' becomes 'step' (synonyms).
'needs an injection of' is an insertion indicator (inserted letters are injected).
'energy' becomes 'e' (physics symbol as in E=mc2).
'step' placed around 'e' is 'STEEP'.
(Other definitions for steep that I've seen before include "Lofty" , "Sharply inclined" , "Rapidly rising" , "over the top" , "Rising sharply" .)