On my back in a sense unknown (9)
I believe the answer is:
anonymous
'unknown' is the definition.
(similar in meaning)
'on my back in a sense' is the wordplay.
'back' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'sense' becomes 'nous' (nous is a kind of sense).
'my' in reverse letter order is 'ym'.
'on'+'ym'='onym'
'a'+'nous'='anous'
'onym' inserted into 'anous' is 'ANONYMOUS'.
(Other definitions for anonymous that I've seen before include "Of unknown identity" , "Incognito" , "Unidentified" , "unnamed source" , "Having no known name" .)