Half contribute to lower building, which is critical to airport operations (7,5)
I believe the answer is:
control tower
'which is critical to airport operations' is the definition.
'half contribute to lower building' is the wordplay.
'half' indicates taking half.
'building' indicates anagramming the letters.
'contribute' with half the letters taken is 'contr'.
'to'+'lower'='tolower'
'tolower' is an anagram of 'oltower'.
'contr'+'oltower'='CONTROL TOWER'
(Other definitions for control tower that I've seen before include "Airport command hub" , "where landing can be supervised" , "Airport building" , "Airport's command centre" , "Airport command point" .)