15 down
Students
about
to
have
sex
endlessly
–
nurse
providing
some
sort
of
protection
(9)
Students about to have sex endlessly – nurse providing some sort of protection (9)
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The definition is at the end of the clue.
How the clue works
Look at providing some sort of protection at the end — that's your definition. NUS (the National Union of Students) gets reversed by "about" to give you SUN, then SCREW (have sex) drops its final letter "endlessly" to leave SCRE, and EN (enrolled nurse) brings up the rear — so you're building SUN plus SCRE plus EN.