Knock true form of ecstasy (7)
I believe the answer is:
rapture
'ecstasy' is the definition.
(I know that ecstasy can be written as rapture)
'knock true form' is the wordplay.
'knock' becomes 'rap' (synonyms).
'form' indicates an anagram (another form of the letters).
'true' anagrammed gives 'ture'.
'rap'+'ture'='RAPTURE'
'of' is the link.
(Other definitions for rapture that I've seen before include "State of bliss and euphoria" , "Bliss, intense delight" , "transport" , "modified for Nanki-Poo?" , "Feeling of bliss, elation" .)