Composed small note to girlfriend (6)
I believe the answer is:
sedate
'composed' is the definition.
(thesaurus)
'small note to girlfriend' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'note' becomes 'e' (musical note).
'to girlfriend' becomes 'date' (I've seen this in other clues).
's'+'e'+'date'='SEDATE'
(Other definitions for sedate that I've seen before include "Calm and demure" , "Give calming drug to" , "Calm or spectral appartition" , "Teased into being calm" , "Administer a calming drug" .)