Seat on plane, mine next to a knob (7)
I believe the answer is:
cockpit
'seat on plane' is the definition.
'cockpit' can be an answer for 'seat' (cockpit is a kind of seat). I am not certain of the 'on plane' bit.
'mine next to a knob' is the wordplay.
'mine' becomes 'pit' (down the pit).
'next to' says to put letters next to each other.
'a knob' becomes 'cock' (I am not sure about this - if you are sure you should believe this answer much more).
'pit' after 'cock' is 'COCKPIT'.
Can you help me to learn more?
(Other definitions for cockpit that I've seen before include "Driving seat" , "In a plane, the pilot sits in this space" , "Aircraft pilot's compartment" , "Air pilot's compartment" , "not suitable for a hen party" .)