He's about to call up a poet (7)
I believe the answer is:
shelley
'a poet' is the definition.
(Percy Bysshe Shelley 1792-1822)
'he's about to call up' is the wordplay.
'about' is an anagram indicator.
'to call' becomes 'yell' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'yell' back-to-front is 'lley'.
'hes' with letters rearranged gives 'she'.
'she'+'lley'='SHELLEY'
(Other definitions for shelley that I've seen before include "English Romantic poet" , "Author of Frankenstein" , "Mary - - (Gothick writer)" , "Percy Bysshe . . . . . . ., great English poet" , "Ode to the West Wind poet" .)