It's heavy though possibly a light attachment (4)
I believe the answer is:
lead
'it's heavy' is the definition.
(thesaurus)
'possibly a light attachment' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'light' could be 'led' (abbreviation for light emitting diode) and 'led' is located in the leftover letters.
No letters remain.
This explanation may well be incorrect...
'though' acts as a link.
Can you help me to learn more?
(Other definitions for lead that I've seen before include "Be ahead of - guide" , "1 down, symbol Pb" , "First put down spade, say" , "Call the shots" , "most important story" .)