Coming down to undress on runway (7,5)
I believe the answer is:
landing strip
'runway' is the definition.
(I've seen this before)
'coming down to undress on' is the wordplay.
'coming down' becomes 'landing' (I've seen this before).
'to undress' becomes 'strip' ('strip' can be a synonym of 'undress').
'on' means one lot of letters go next to another.
'landing'+'strip'='LANDING-STRIP'
(Other definitions for landing strip that I've seen before include "Where fighter comes in" , "Small airfield" , "Runway" , "A basic airfield" , "flights end here)" .)