Lookout cross, went mad (5-4)
I believe the answer is:
crows-nest
'lookout' is the definition.
(I've seen this before)
'cross went mad' is the wordplay.
'mad' is an anagram indicator.
'cross'+'went'='crosswent'
'crosswent' anagrammed gives 'CROWS-NEST'.
(Other definitions for crows-nest that I've seen before include "Lookout platform high on ship's mast (4'1, 4)" , "Look-out platform on sailing ship, or black bird's home" , "Ship's lookout point" , "Ship lookout's position" , "Masthead platform" .)