Think Calliope is one (4)
I believe the answer is:
muse
I believe this clue is a double definition.
'think' is the first definition.
(to muse on something is to think about it)
'calliope is one' is the second definition.
(I know that Calliope is Muse)
(Other definitions for muse that I've seen before include "Think - goddess of the liberal arts" , "Contemplate -- a goddess of the liberal arts" , "Cogitate" , "Inspiring goddess" , "Ponder in meditative way" .)