Louse-egg chosen to be pettily criticised (9)
I believe the answer is:
nitpicked
'pettily criticised' is the definition.
'nitpicked' can be an answer for 'criticised' (nitpicking is a kind of criticizing). I'm not sure about the 'pettily' bit.
'louse-egg chosen' is the wordplay.
'louse-egg' becomes 'nit' (I've seen this before).
'chosen' becomes 'picked' (picking is a kind of choosing).
'nit'+'picked'='NITPICKED'
'to be' is the link.
Can you help me to learn more?
(Other definitions for nitpicked that I've seen before include "Quibbled" , "Made petty criticisms" .)