Head over to Spain by air (4)
I believe the answer is:
tune
'air' is the definition.
('tune' can be a synonym of 'air')
'head over to spain' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'over' says the letters should be written in reverse.
'to' says to put letters next to each other.
'spain' becomes 'e' (abbreviation for Espana**).
'nut' reversed gives 'tun'.
'tun'+'e'='TUNE'
'by' is the link.
(Other definitions for tune that I've seen before include "temper" , "'He who pays the piper calls the ...'" , "strain" , "Improve quality" , "Air, melody" .)