Imagine visitors wasting time (5)
I believe the answer is:
guess
'imagine' is the definition.
(I know that imagine can be written as guess)
'visitors wasting time' is the wordplay.
'visitors' becomes 'guests' (guest is a kind of visitor).
'wasting' indicates named letters should be taken away.
'time' becomes 't' (abbreviation).
'guests' with 't' taken out is 'GUESS'.
(Other definitions for guess that I've seen before include "Conjecture - estimate" , "Estimate; suppose" , "Speculation" , "shot?" , "Make stab at" .)