A number put on tissue (6)
I believe the answer is:
tendon
'tissue' is the definition.
(tendon is a kind of tissue)
'a number put on' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'put on' becomes 'don' (as in donning or putting on an item of clothing).
'ten'+'don'='TENDON'
(Other definitions for tendon that I've seen before include "personal connecting link" , "It's attaches tissue to bone" , "Strong cord of bodily tissue" , "Sinew, maybe 3" , "Tissue connecting a muscle with its bony attachment" .)