It's mighty fine to go round a street endlessly (3,4)
I believe the answer is:
oak tree
'it's mighty' is the definition.
(oak trees are proverbially strong)
'fine to go round a street endlessly' is the wordplay.
'fine' becomes 'ok' (synonyms).
'to go round' means one lot of letters goes inside another.
'endlessly' says to take the centre (the ends of the words are removed).
The central letters of 'street' are 'tree'.
'ok' enclosing 'a' is 'oak'.
'oak'+'tree'='OAK-TREE'
(Other definitions for oak tree that I've seen before include "Acorn producer" , "Source of acorns" , "wood source?" , "famous royal hideout" , "royal hiding place evidently" .)