Join with good heart in duet about Love's Beginning (4)
I believe the answer is:
glue
'join' is the definition.
(I know that glue is a more specific form of the action fasten)
'good heart in duet about love's beginning' is the wordplay.
'good' becomes 'g' (abbreviation).
'heart in' says to take the centre.
'about' indicates an anagram.
'beginning' suggests taking the first letters.
The middle letters of 'duet' are 'ue'.
The first letter of 'loves' is 'l'.
'ue' anagrammed with 'l' makes 'lue'.
'g'+'lue'='GLUE'
'with' is the link.
(Other definitions for glue that I've seen before include "agent with sticking power" , "Adhesive substance" , "Stuff that sticks firmly" , "Sticky substance" , "Sticker" .)