Letter pi in cast steel (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that epistle is a type of letter)
'pi in cast steel' is the wordplay.
'in' means one lot of letters goes inside another.
'cast' indicates an anagram (letters are dispersed or cast around).
'steel' anagrammed gives 'estle'.
'pi' put into 'estle' is 'EPISTLE'.
(Other definitions for epistle that I've seen before include "screed" , "(E.g. Paul's) letter" , "Romans perhaps" , "Writing" , "may be written off" .)