Meet up and bristle (4)
I believe the answer is:
teem
'bristle' is the definition.
(thesaurus)
'meet up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'meet' in reverse letter order is 'TEEM'.
'and' is the link.
(Other definitions for teem that I've seen before include "Move in large numbers" , "Rain hard" , "stream" , "Come down hard" , "Abound, swarm with" .)