Lining up is unique, e.g. when dancing (8)
I believe the answer is:
queueing
'lining up' is the definition.
The definition and answer can be both to do with motion as well as being present participle verbs.
Perhaps there's an association between them I don't understand?
'unique e g when dancing' is the wordplay.
'when dancing' indicates anagramming the letters (letters dance around).
'unique'+'e'+'g'='uniqueeg'
'uniqueeg' is an anagram of 'QUEUEING'.
'is' is the link.
Can you help me to learn more?
(Other definitions for queueing that I've seen before include "Standing in line" , "Waiting in line" .)