Brexiteer at heart, caught in very Remain atmosphere (4)
I believe the answer is:
vibe
'atmosphere' is the definition.
(I've seen this before)
'brexiteer at heart caught in very remain' is the wordplay.
'at heart' says to take the centre.
'caught in' means one lot of letters goes inside another.
'very' becomes 'v' (abbreviation).
'remain' becomes 'be' (I've seen this in other clues).
The central letter of 'brexiteer' is 'i'.
'v'+'be'='vbe'
'i' inserted into 'vbe' is 'VIBE'.
(Other definitions for vibe that I've seen before include "Aura experienced instinctively" , "Aura given off by person/place" , "Perceptible aura" , "Atmosphere, feeling" , "Aura (colloq.)" .)