Since start of Brexit, Spain imports sauce, strangely (7)
I believe the answer is:
because
'since' is the definition.
(I've seen this before)
'start of brexit spain imports sauce strangely' is the wordplay.
'start of' indicates taking the first letters.
'spain' becomes 'e' (abbreviation for Espana).
'imports' means one lot of letters goes inside another (inserted letters are imported).
'strangely' indicates anagramming the letters.
The initial letter of 'brexit' is 'b'.
'b'+'e'='be'
'sauce' with letters rearranged gives 'ecaus'.
'be' placed around 'ecaus' is 'BECAUSE'.
(Other definitions for because that I've seen before include "Since" , "For the reason that" , "On account of" , "Beginning to explain" , "Owing to" .)