Long stitch may fasten a carpet (4)
I believe the answer is:
tack
This could be a double definition.
'long stitch' is the first definition.
'tack' can be an answer for 'stitch' (I have seen 'Temporary stitch ' mean 'tack' so perhaps 'stitch' could also mean 'tack'). I am not certain of the 'long' bit.
'fasten a carpet' is the second definition.
'tack' can be an answer for 'fasten' (tacking is a kind of fastening). I'm not sure about the 'a carpet' bit.
'may' acts as a link.
Can you help me to learn more?
(Other definitions for tack that I've seen before include "Nail; tasteless material" , "Riding saddles, bridles etc" , "Small nail with broad head" , "Stickiness" , "Short, flat-headed nail" .)