Fibrous tissue is held up, previously unseen (5)
I believe the answer is:
sinew
'fibrous tissue' is the definition.
(sinew is a kind of fibrous tissue)
'is held up previously unseen' is the wordplay.
'held up' says the letters should be written backwards.
'previously unseen' becomes 'new' (I've seen this in another clue).
'is' written backwards gives 'si'.
'si'+'new'='SINEW'
(Other definitions for sinew that I've seen before include "Gristle" , "Tendon - ligament" , "power source" , "fibre" , "Muscular strength" .)