Sad if solemn, a British dependency (4,2,3)
I believe the answer is:
isle of man
'british dependency' is the definition.
'sad if solemn a' is the wordplay.
'sad' is an anagram indicator.
'if'+'solemn'+'a'='ifsolemna'
'ifsolemna' is an anagram of 'ISLE OF MAN'.
(Other definitions for isle of man that I've seen before include "Its cats are said to be lacking tails" , "Island in 33 Across where cats have no tails" , "Island in the Irish Sea" , "directed by Douglas" , "British Crown possession" .)