Fellow - former schoolfellow - showing attachment to watch (3)
I believe the answer is:
fob
'attachment to watch' is the definition.
'fellow former schoolfellow' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'former schoolfellow' becomes 'ob' (abbreviation for old boy).
'f'+'ob'='FOB'
'showing' acts as a link.
(Other definitions for fob that I've seen before include "Chain attached to watch" , "Ornament worn on a watch-chain" , "Small watch pocket" , "Ornament worn on watch chain" , "It is worn hanging from a watch-chain" .)