Install new shed with external bolt for farm animals (9)
I believe the answer is:
livestock
'farm animals' is the definition.
(I've seen this before)
'install new shed with external bolt for' is the wordplay.
'install' becomes 'invest' (investing is a kind of installing).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'shed' suggests deleting specific letters ('shed' as in 'cut away').
'with external' means one lot of letters goes inside another.
'bolt for' becomes 'lock' (I am not sure about this - if you are sure you should believe this answer much more).
'invest' with 'n' taken away is 'ivest'.
'ivest' placed inside 'lock' is 'LIVESTOCK'.
(Other definitions for livestock that I've seen before include "(Domestic) animals in general" , "Animals kept for use or profit" , "Farm animals" .)