Picture mentally (8)
'picture mentally' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for imagine that I've seen before include "that Lennon composed" , "Enigma, I? (anag)" , "Suppose; conceive" , "Picture in mind, envisage" , "Conjure up in mind, visualise" .)
(Other definitions for envisage that I've seen before include "Visualise, imagine" , "Visualise - vine's age (anag)" , "Form a mental image of" , "Contemplate, imagine" , "Form a mental picture of (something)" .)