Peter and Mimi dancing when most watch (5,4)
I believe the answer is:
prime time
'when most watch' is the definition.
(most popular time for TV viewing)
'peter and mimi dancing' is the wordplay.
'and' means one lot of letters go next to another.
'dancing' indicates anagramming the letters.
'peter'+'mimi'='petermimi'
'petermimi' with letters rearranged gives 'PRIME TIME'.
(Other definitions for prime time that I've seen before include "For monks perhaps around dawn" , "when most view" , "Peak viewing period" , "Optimum period for broadcasters" , "When viewing figures are highest" .)