Think of a number (6)
This clue is a double definition.
'think' is the first definition.
(thesaurus)
'a number?' is the second definition.
(figure is a kind of number)
'of' is the link.
(Other definitions for figure that I've seen before include "Build" , "Imagine; integer" , "Shape; amount of money" , "The written symbol of a number" , "form" .)
(Other definitions for imagine that I've seen before include "Picture to oneself" , "Conceive in the mind" , "Reckon" , "Number one" , "Picture in mind, envisage" .)