Immediate tension as aunt cooks (13)
I believe the answer is:
instantaneous
'immediate' is the definition.
('instantaneous' can be similar in meaning to 'immediate')
'tension as aunt cooks' is the wordplay.
'cooks' indicates anagramming the letters.
'tension'+'as'+'aunt'='tensionasaunt'
'tensionasaunt' anagrammed gives 'INSTANTANEOUS'.
(Other definitions for instantaneous that I've seen before include "Occurring in a moment" , "Momentary" , "Done without delay" , "On-the-spot" , "Immediate" .)