Deny failing to save energy? That would be poor (5)
I believe the answer is:
needy
'poor' is the definition.
(similar in meaning)
'deny failing to save energy?' is the wordplay.
'failing' indicates an anagram (a failed spelling of the word).
'to save' indicates putting letters inside.
'energy?' becomes 'e' (physics symbol as in E=mc2).
'deny' is an anagram of 'nedy'.
'nedy' going around 'e' is 'NEEDY'.
'that would be' acts as a link.
(Other definitions for needy that I've seen before include "Penurious" , "Insecure; very poor" , "penniless" , "Poor; wanting affection" , "wanting much" .)