A bit of luck to understand her completely (10)
I believe the answer is:
altogether
'completely' is the definition.
('altogether' can be a synonym of 'completely')
'a bit of luck to understand her' is the wordplay.
'bit of' suggests taking the first letters.
'understand' becomes 'get' (getting is a kind of understanding).
The first letter of 'luck' is 'l'.
'a'+'l'+'to'+'get'+'her'='ALTOGETHER'
(Other definitions for altogether that I've seen before include "Completely, in total" , "Completely, on the whole" , "special to get Hermes trousers" , "totally" , "Entirely" .)