Officer's curiously cool letter to the Spanish (7)
I believe the answer is:
colonel
'officer's' is the definition.
(colonel is a kind of officer)
'curiously cool letter to the spanish' is the wordplay.
'curiously' indicates an anagram.
'letter' becomes 'n' ().
'to the spanish' becomes 'el' ('the' in Spanish).
'cool' with letters rearranged gives 'colo'.
'colo'+'n'+'el'='COLONEL'
(Other definitions for colonel that I've seen before include "No cello (anag.)" , "Rank (below brigadier)" , "Army three downn" , "Bogey perhaps" , "High-ranking officer" .)