A slowing down in network is no common occurrence? (6)
I believe the answer is:
rarity
'no common occurrence?' is the definition.
(rarity can mean a rare event)
'a slowing down in network' is the wordplay.
'slowing down' becomes 'rit' (abbreviation for musical term ritardando).
'in' indicates putting letters inside.
'network' becomes 'RY' (abbreviation for railway).
'a'+'rit'='arit'
'arit' put within 'ry' is 'RARITY'.
'is' acts as a link.
(Other definitions for rarity that I've seen before include "something out of the ordinary" , "Something very scarce and often valuable" , "Very unusual occurrence" , "Find" , "Something valued for its scarceness" .)