Yearn for each other (4)
I believe the answer is:
ache
'yearn' is the definition.
(synonyms)
'each other' is the wordplay.
'other' indicates anagramming the letters.
'each' with letters rearranged gives 'ACHE'.
'for' is the link.
(Other definitions for ache that I've seen before include "Cause pain" , "Play up" , "agony" , "Continuous hurt" , "Hurt; yearn" .)