Landowner involved in tactical air defence (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'involved in tactical air defence' is the wordplay.
'involved in' indicates the answer is hidden within the clue.
'LAIRD' can be found hidden inside 'tactical air defence'.
(Other definitions for laird that I've seen before include "Proprietor" , "Scottish owner of large estate" , "Man owning land" , "Scottish estate-owner" , "Scottish squire" .)