Betting engineers are on energy binge (5)
I believe the answer is:
spree
'binge' is the definition.
(I've seen this in another clue)
'betting engineers are on energy' is the wordplay.
'betting' becomes 'sp'.
'engineers' becomes 're' (Royal Engineers).
'are on' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'sp'+'re'+'e'='SPREE'
(Other definitions for spree that I've seen before include "Spell of unrestrained activity" , "Unrestrained bout" , "Drinking bout" , "Wild spell of spending or drinking" , "Wild shopping expedition" .)