Immediate sensation, aunt dancing (13)
I believe the answer is:
instantaneous
'immediate' is the definition.
('instantaneous' can be similar in meaning to 'immediate')
'sensation aunt dancing' is the wordplay.
'dancing' indicates an anagram.
'sensation'+'aunt'='sensationaunt'
'sensationaunt' with letters rearranged gives 'INSTANTANEOUS'.
(Other definitions for instantaneous that I've seen before include "Immediate" , "Occurring in a moment" , "Momentary" , "Done in a moment" , "On-the-spot" .)