Fasten it back with energy (3)
I believe the answer is:
tie
'fasten' is the definition.
(I know that fasten is a more specific form of attach)
'it back with energy' is the wordplay.
'back' is a reversal indicator.
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'it' written backwards gives 'ti'.
'ti'+'e'='TIE'
(Other definitions for tie that I've seen before include "restraint" , "No-win situation" , "Almost level" , "Lace" , "Get even" .)