Knowledge of kraken half-formed (3)
I believe the answer is:
ken
'knowledge' is the definition.
(ken is a kind of knowledge)
'kraken half-formed' is the wordplay.
'half-formed' indicates taking half.
'kraken' with half the letters taken is 'KEN'.
'of' acts as a link.
(Other definitions for ken that I've seen before include "Identify; -- Livingstone" , "Understanding - boy's name" , "Awareness" , "Notice" , "One's range of knowledge" .)