Naval unit die in action, being so old (12)
I believe the answer is:
antediluvian
'so old' is the definition.
'antediluvian' can be an answer for 'old' (similar in meaning). I'm not sure about the 'so' bit.
'naval unit die in action' is the wordplay.
'in action' is an anagram indicator (I've seen this in other clues).
'naval'+'unit'+'die'='navalunitdie'
'navalunitdie' is an anagram of 'ANTEDILUVIAN'.
'being' is the link.
Can you help me to learn more?
(Other definitions for antediluvian that I've seen before include "Before the flood; ancient" , "Very old (before the Flood)" , "before the water's bubbled up" , "Very old-fashioned (from before the Flood?)" .)