Dad conserving energy as climber (3)
I believe the answer is:
pea
'climber' is the definition.
(I've seen this before)
'dad conserving energy' is the wordplay.
'dad' becomes 'pa' (synonyms).
'conserving' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'pa' placed around 'e' is 'PEA'.
'as' acts as a link.
(Other definitions for pea that I've seen before include "It grows in a pod" , "Leguminous vegetable" , "Small vegetable" , "Little green vegetable" , "Edible seed from pod" .)