Accidentally purchase missing uranium, with luck (2,6)
I believe the answer is:
by chance
'accidentally' is the definition.
(synonyms)
'purchase missing uranium with luck' is the wordplay.
'purchase' becomes 'buy' (synonyms).
'missing' suggests deleting specific letters.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'with' says to put letters next to each other.
'luck' becomes 'chance' (synonyms).
'buy' with 'u' taken out is 'by'.
'by'+'chance'='BY CHANCE'
(Other definitions for by chance that I've seen before include "As it happened, without being planned" , "Without advance planning" , "Fortuitously" , "Accidentally, not planned" , "unwittingly" .)