Eggs on, very assiduously at first (3)
I believe the answer is:
ova
'eggs' is the definition.
(term for egg cells)
'on very assiduously at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'on very assiduously' is 'OVA'.
(Other definitions for ova that I've seen before include "Eggs; sounds like above" , "targets for male seed?" , "Latin egg-cells" , "Female reproductive cells" , "Likely end for Russian Fed cup players" .)