Test by Henry securing university entrance (5)
I believe the answer is:
mouth
'entrance' is the definition.
(eg the mouth of a cave)
'test by henry securing university' is the wordplay.
'test' becomes 'MOT' (as in an MOT test for a vehicle).
'by' means one lot of letters go next to another.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'securing' means one lot of letters goes inside another.
'university' becomes 'u' (abbreviation for university).
'mot'+'h'='moth'
'moth' placed around 'u' is 'MOUTH'.
(Other definitions for mouth that I've seen before include "chops" , "Opening of a bay" , "Oral cavity" , "End of river" , "Sauce" .)