Bird seen by five hundred — that's a lot of people! (5)
I believe the answer is:
crowd
'lot of people' is the definition.
'crowd' can be an answer for 'lot' (thesaurus). I am not sure about the 'of people' bit.
'bird seen by five hundred' is the wordplay.
'bird' becomes 'crow' (crow is a kind of bird).
'seen by' says to put letters next to each other.
'five hundred' becomes 'd' (Roman numeral for 500).
'crow'+'d'='CROWD'
'that's a' is the link.
Can you help me to learn more?
(Other definitions for crowd that I've seen before include "assembly" , "Horde" , "Multitude" , "Social set" , "See 1" .)