Sperm densities regularly start to rise in academic session (8)
I believe the answer is:
semester
'academic session' is the definition.
(I've seen this in another clue)
'sperm densities regularly start to rise' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'start to' suggests taking the first letters.
The first letter of 'rise' is 'r'.
'sperm'+'densities'='spermdensities'
The alternate letters of 'spermdensities' are 'semeste'.
'semeste'+'r'='SEMESTER'
'in' acts as a link.
(Other definitions for semester that I've seen before include "Academic half-year term" , "Half-year university session" , "A term in American university" , "Steers me towards university term" , "study period" .)