Baltic golf learners welcoming a bunch of spies (7)
I believe the answer is:
glacial
'baltic' is the definition.
(both can mean very cold)
'golf learners welcoming a bunch of spies' is the wordplay.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'learners' becomes 'LL' (two abbreviations for learner).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'bunch of spies' becomes 'CIA' (Central Intelligence Agency).
'a'+'cia'='acia'
'll' enclosing 'acia' is 'lacial'.
'g'+'lacial'='GLACIAL'
(Other definitions for glacial that I've seen before include "Freezing - unfriendly" , "Cold and icy" , "Of slow moving ice masses" , "Relating to ice" , "Frigid" .)