Lighthouse overseers, three working in British navy, standing beside tree (5,8)
I believe the answer is:
elder brethren
'lighthouse overseers three working' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'british navy standing beside tree' is the wordplay.
'british navy' becomes 'brethren' (I can't justify this - if you can you should give a lot more credence to this answer).
'standing beside' says to put letters next to each other.
'tree' becomes 'elder' (elder is a kind of tree).
'brethren' put after 'elder' is 'ELDER BRETHREN'.
'in' is the link.
Can you help me to learn more?
(Other definitions for elder brethren that I've seen before include "Governing members of the Corporation of Trinity House" , "Charitable governors" .)