Length of service (4)
'length of service' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tenure that I've seen before include "Term of office" , "The holding of office or land" , "Retune (anag.)" , "Right to keep job" , "Way of holding" .)
(Other definitions for term that I've seen before include "Word; limited period" , "Hilary for one" , "A fixed or limited time - or an expression" , "Expression, or period in which law court holds sessions" , "what being 9 [PREGNANT] leads to?" .)