A lot of old headphones save energy (6)
I believe the answer is:
oceans
'a lot' is the definition.
(I've seen this in another clue)
'old headphones save energy' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is found within the answer.
'headphones' could be 'cans' (I've seen this before) and 'cans' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for oceans that I've seen before include "big seas" , "expanses?" , "Bodies of salt water" , "Southern Indian" , "Large seas" .)