Established in low delta (5)
I believe the answer is:
based
'established' is the definition.
('base' can be a synonym of 'establish')
'low delta' is the wordplay.
'low' becomes 'base' (synonyms).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'base'+'d'='BASED'
'in' is the link.
(Other definitions for based that I've seen before include "Located" , "foundation" , "(Of aircraft) stationed" , "Positioned" , "Founded, grounded" .)