Structures in the form of a spire! (5)
I believe the answer is:
piers
'structures' is the definition.
(pier is a kind of structure)
'form of a spire' is the wordplay.
'form of' indicates an anagram (another form of the same letters).
'spire' anagrammed gives 'PIERS'.
'in the' is the link.
(Other definitions for piers that I've seen before include "Supports for bridge spans" , "Jetties" , "Landing stages" , "Old ploughman" .)