University probing Drosophila, mostly with success (8)
I believe the answer is:
fruitful
'with success' is the definition.
'university probing drosophila mostly' is the wordplay.
'university' becomes 'u'.
'probing' is an insertion indicator (probe can mean to make a hole in).
'drosophila' becomes 'fruitfly' (drosophila is a type of fruit fly).
'mostly' means to remove the last letter.
'fruitfly' with its last letter taken away is 'fruitfl'.
'u' going within 'fruitfl' is 'FRUITFUL'.
(Other definitions for fruitful that I've seen before include "rich" , "Fertile, productive" , "having plenty of dates?" , "profitable" , "Fertile, abundant" .)