Knock, finding the head attentive (4)
I believe the answer is:
rapt
'attentive' is the definition.
(similar in meaning)
'knock finding the head' is the wordplay.
'knock' becomes 'rap' (synonyms).
'finding' is a charade indicator (letters next to each other).
'head' says to take the initial letters.
The initial letter of 'the' is 't'.
'rap'+'t'='RAPT'
(Other definitions for rapt that I've seen before include "Spellbound in part" , "Ecstatic - engrossed" , "Held spellbound" , "riveted" , "Enraptured, entranced" .)