Mark joins the Spanish officer (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'mark joins the spanish' is the wordplay.
'mark' becomes 'colon' (colon is a kind of mark).
'joins' is a charade indicator (letters next to each other).
'the spanish' becomes 'el' ('the' in Spanish).
'colon'+'el'='COLONEL'
(Other definitions for colonel that I've seen before include "soldier" , "'No cello for this officer, strangely (7)'" , "Army three downn" , "High-ranking officer" , "Army officer ranking below a brigadier" .)