Large crowd in area of Plymouth crossing road (5)
I believe the answer is:
horde
'large crowd' is the definition.
(I've seen this in another clue)
'area of plymouth crossing road' is the wordplay.
'area of plymouth' becomes 'hoe' (I can't explain this - if you can you should give a lot more credence to this answer).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'road' becomes 'rd' (abbreviation).
'hoe' placed around 'rd' is 'HORDE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for horde that I've seen before include "Vast crowd" , "Large army" , "Large group of people" , "A swarm or multitude" , "Throng, mass" .)