Taxis to join up, moving about in proximity (13)
I believe the answer is:
juxtaposition
'proximity' is the definition.
(I have seen 'Close proximity ' mean 'juxtaposition' so perhaps 'proximity' could also mean 'juxtaposition')
'taxis to join up moving about' is the wordplay.
'moving about' indicates an anagram.
'taxis'+'to'+'join'+'up'='taxistojoinup'
'taxistojoinup' anagrammed gives 'JUXTAPOSITION'.
'in' acts as a link.
(Other definitions for juxtaposition that I've seen before include "Adjacency" , "Placing of things close together" , "Close proximity" , "Closeness allowing for analysis of contrast" , "Adjacent placement" .)