On foot of finding one of them in Wellington
I believe the answer is:
instep
'on foot of finding one of them' is the definition.
I can't judge whether this definition defines the answer.
'in wellington' is the wordplay.
'wellington' becomes 'step' (I can't justify this - if you can you should believe this answer much more).
'in'+'step'='INSTEP'
Can you help me to learn more?
(Other definitions for instep that I've seen before include "Foot arch - conforming" , "Part of foot between toes and ankle" , "Part of foot under spinet" , "Sir Jack --, famous English cricketer" , "downtrodden area" .)