Fanatical airmen deployed overseas (11)
I believe the answer is:
ultramarine
'overseas' is the definition.
(thesaurus)
'fanatical airmen deployed' is the wordplay.
'fanatical' becomes 'ultra' (I've seen this in another clue).
'deployed' indicates an anagram.
'airmen' with letters rearranged gives 'marine'.
'ultra'+'marine'='ULTRAMARINE'
(Other definitions for ultramarine that I've seen before include "Vivid blue" , "Blue pigment originally made from lapis lazuli" , "See a colour" , "Deep blue pigment" , "Brilliant deep blue" .)