About to interrupt mother? Imagine (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'about to interrupt mother?' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to interrupt' means one lot of letters goes inside another.
'mother?' becomes 'dam' (female parent of an animal).
're' inserted inside 'dam' is 'DREAM'.
(Other definitions for dream that I've seen before include "Cherished hope; fantasy" , "dozy state" , "Fantasy; ideal" , "Switch off" , "Illusion" .)