Perch or pole fixed in foundation (5)
I believe the answer is:
roost
'perch' is the definition.
(synonyms)
'pole fixed in foundation' is the wordplay.
I cannot really understand how this works, but
'pole' could be 's' (abbreviation for South, as in South Pole) and 's' is present in the answer.
'foundation' could be 'root' (I've seen this in another clue) and 'root' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'or' is the link.
Can you help me to learn more?
(Other definitions for roost that I've seen before include "Perch where bird sleeps" , "It's sometimes ruled" , "Temporary resting-place" , "Torso (anag)" , "Where birds rest at night" .)