Rest of Belgium somewhat encapsulating Spain (8)
I believe the answer is:
breather
'rest' is the definition.
(breather is a kind of rest)
'belgium somewhat encapsulating spain' is the wordplay.
'belgium' becomes 'b' (international vehicle registration code).
'somewhat' becomes 'rather' (I've seen this before).
'encapsulating' is an insertion indicator (I've seen this in other clues).
'spain' becomes 'e' (abbreviation for Espana).
'rather' placed around 'e' is 'reather'.
'b'+'reather'='BREATHER'
'of' is the link.
(Other definitions for breather that I've seen before include "Short rest break" , "lung, possibly?" , "Short respite" , "lung or > liver?" , "state" .)