How's your father after little brother sent back cycle? (5)
I believe the answer is:
orbit
'cycle?' is the definition.
(thesaurus)
'how's your father after little brother sent back' is the wordplay.
'how's your father' becomes 'it' (both are euphemisms for sex).
'after' is a charade indicator (letters next to each other).
'little brother' becomes 'bro'.
'sent back' shows that the letters should be reversed in order.
'bro' in reverse letter order is 'orb'.
'it' after 'orb' is 'ORBIT'.
(Other definitions for orbit that I've seen before include "area of activity" , "Circle - influence" , "Path of a planet or satellite" , "Sphere of activity" , "Planet's path" .)