Start with money to maintain Anglesey's principal harbour (9)
I believe the answer is:
entertain
'harbour' is the definition.
(synonyms)
'start with money to maintain anglesey's principal' is the wordplay.
'start' becomes 'enter' (I am not sure about this - if you are sure you should believe this answer much more).
'with money' becomes 'tin' (I've seen this in other clues).
'to maintain' means one lot of letters goes inside another.
'principal' suggests taking the first letters.
The initial letter of 'anglesey' is 'a'.
'tin' going around 'a' is 'tain'.
'enter'+'tain'='ENTERTAIN'
Can you help me to learn more?
(Other definitions for entertain that I've seen before include "Think about" , "consider" , "Act as host" , "Amuse or take into consideration" , "what the party host will do" .)