Mean fellow conserving energy (5)
I believe the answer is:
cheap
'mean' is the definition.
(I've seen this before)
'fellow conserving energy' is the wordplay.
'fellow' becomes 'chap' ('chap' can be a synonym of 'fellow'**).
'conserving' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mch).
'chap' placed around 'e' is 'CHEAP'.
(Other definitions for cheap that I've seen before include "Inexpensive or tasteless" , "Vulgar" , "Budget -cheesy" , "Not expensive - trashy" , "Contemptible" .)