Wide, rotten boat full of animals covers miles (3,3,4)
I believe the answer is:
off the mark
'animals covers miles' is the definition.
I know nothing about this answer so I can't tell whether this works.
'wide rotten boat full of' is the wordplay.
'wide' becomes 'them' (I can't justify this - if you can you should give a lot more credence to this answer).
'rotten' becomes 'off' (rotten food is 'off').
'boat' becomes 'ark' (ark is a kind of boat).
'full of' is an insertion indicator.
'off'+'ark'='offark'
'them' put inside 'offark' is 'OFF THE MARK'.
Can you help me to learn more?
(Other definitions for off the mark that I've seen before include "Having started" , "Inaccurate; getting first run" , "that's not right" , "under a misapprehension" .)