Fortress on secure headland (8)
I believe the answer is:
fastness
'fortress' is the definition.
(thesaurus)
'on secure headland' is the wordplay.
'on' is a charade indicator (letters next to each other).
'secure' becomes 'fast' ('fast' can be similar in meaning to 'secure'**).
'headland' becomes 'ness' (I've seen this before).
'fast'+'ness' is 'FASTNESS'.
(Other definitions for fastness that I've seen before include "Keep" , "speed?" , "Strongly fortified defensive place" , "Secure refuge" , "quality of not fading" .)