Wind could be constant energy source (4)
I believe the answer is:
coil
'wind' is the definition.
(I know that coil is a is a more specific form of the action wind)
'constant energy source' is the wordplay.
'constant' becomes 'C' (common symbol in maths for an unknown constant).
'energy source' becomes 'oil' (I've seen this in another clue).
'c'+'oil'='COIL'
'could be' is the link.
(Other definitions for coil that I've seen before include "Convolute" , "Wind round and round in loops" , "Loop - conductor" , "Wind (ropes) - gather (hair)" , "Helix" .)