I'd clap, awfully tranquil (6)
I believe the answer is:
placid
'tranquil' is the definition.
(thesaurus)
'i'd clap awfully' is the wordplay.
'awfully' indicates anagramming the letters.
'id'+'clap'='idclap'
'idclap' with letters rearranged gives 'PLACID'.
(Other definitions for placid that I've seen before include "Unconcerned" , "Calm and not easily upset" , "or is it an American one?" , "Collected" , "Cool and unruffled" .)