Nudge energy arc to contain luminance (5)
I believe the answer is:
elbow
'nudge' is the definition.
('elbow' can be a synonym of 'nudge')
'energy arc to contain luminance' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'arc' becomes 'bow' ('bow' can be a synonym of 'arc').
'to contain' means one lot of letters goes inside another.
'luminance' becomes 'l' (physics abbreviation).
'e'+'bow'='ebow'
'ebow' going around 'l' is 'ELBOW'.
(Other definitions for elbow that I've seen before include "Major arm joint" , "Limb hinge" , "Reject" , "Below (anag.)" , "sharp turn" .)