Fancy mother carrying on! (5)
I believe the answer is:
dream
'fancy' is the definition.
(thesaurus)
'mother carrying on' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'carrying' is an insertion indicator.
'on' becomes 're' ('on' can be mean 'with reference to').
'dam' enclosing 're' is 'DREAM'.
(Other definitions for dream that I've seen before include "Sleeping experience" , "Reverie or nightmare" , "Illusion" , "Fantasy; imagine" , "Cherished hope or ideal" .)