Knock to get the head attentive (4)
I believe the answer is:
rapt
'attentive' is the definition.
(similar in meaning)
'knock to get the head' is the wordplay.
'knock' becomes 'rap' (synonyms).
'to get' says to put letters next to each other.
'head' says to take the initial letters.
The first letter of 'the' is 't'.
'rap'+'t'='RAPT'
(Other definitions for rapt that I've seen before include "Part (anag.)" , "Engrossed, spellbound" , "Part being transported" , "'Transported, entranced (4)'" , "Part of being intent and absorbed" .)