Leant out, energy in that hidden warmth (6,4)
I believe the answer is:
latent heat
'hidden warmth' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'leant out energy in that' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'e' inserted within 'that' is 'theat'.
'leant' anagrammed gives 'laten'.
'laten'+'theat'='LATENT HEAT'
Can you help me to learn more?
(Other definitions for latent heat that I've seen before include "Energy needed to cause a change of state, eg, solid to vapour" , "Natal teeth (anag.)" .)