Unexpectedly, come across leaders of this operation after argument (3,4)
I believe the answer is:
run into
'unexpectedly come' is the definition.
'run into' can be an answer for 'come' (thesaurus). I'm not sure about the 'unexpectedly' bit.
'across leaders of this operation after argument' is the wordplay.
'across' means one lot of letters go next to another (across can mean on).
'leaders of' says to take the initial letters.
'after argument' becomes 'runin' (I've seen this before. I am not sure about the 'after' bit.).
The first letters of 'this operation' is 'to'.
'to' after 'runin' is 'RUN INTO'.
Can you help me to learn more?
(Other definitions for run into that I've seen before include "Encounter (someone)" , "Meet (a person) by chance" , "charge?" , "'Meet by chance (3,4)'" , "Come across, encounter" .)