Train needs energy except for reversing (4)
I believe the answer is:
tube
'train' is the definition.
(thesaurus)
'energy except for reversing' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'except' becomes 'but' (I've seen this before).
'for reversing' is a reversal indicator.
'e'+'but'='ebut'
'ebut' in reverse letter order is 'TUBE'.
'needs' acts as a link.
(Other definitions for tube that I've seen before include "Valve" , "Hollow cylindrical object" , "appearing tinny?" , "Can of beer (colloquial)" , "Underground transport" .)