This writer's son, twitching person aware of another world? (6)
I believe the answer is:
mystic
'person aware of another world?' is the definition.
(someone aware of mystical things)
'this writer's son twitching' is the wordplay.
'this writer's' becomes 'my' (belonging to the person who set the crossword).
'son' becomes 's' (genealogical abbreviation for son).
'twitching' becomes 'tic' (tic is a kind of twitching).
'my'+'s'+'tic'='MYSTIC'
(Other definitions for mystic that I've seen before include "Of hidden or symbolic meaning as in religion" , "Believer in truths beyond tangible proof" , "Allegorical; sacredly obscure" , "Paranormal, mysterious" , "Relating to occult rites" .)