First of geese landing in lake group together (5)
I believe the answer is:
merge
'group together' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.
'first of geese landing in lake' is the wordplay.
'first of geese landing' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'lake' becomes 'mere' (mere is a kind of lake).
'g' placed within 'mere' is 'MERGE'.
Can you help me to learn more?
(Other definitions for merge that I've seen before include "Join into one" , "Combine to form a single entity" , "Combine, coalesce" , "Amalgamate" , "Come together as one" .)