Mount collage, initially with border (5)
I believe the answer is:
climb
'mount' is the definition.
('climb' can be a synonym of 'mount')
'collage initially with border' is the wordplay.
'initially' indicates taking the first letters.
'with' says to put letters next to each other.
'border' becomes 'limb' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'collage' is 'c'.
'c'+'limb'='CLIMB'
Can you help me to learn more?
(Other definitions for climb that I've seen before include "Mount, ascend" , "Scale or clamber" , "Rise (in value, status)" , "Go higher up" , "Get a rise" .)