Head on lap, it's me in cap (5)
I believe the answer is:
limit
'cap' is the definition.
(I know that cap is a more specific form of the action limit)
'head on lap it's me in' is the wordplay.
'head on' indicates taking the first letters.
'me' becomes 'mi' (both are spellings of the musical note).
'in' means one lot of letters goes inside another.
The first letter of 'lap' is 'l'.
'it' enclosing 'mi' is 'imit'.
'l'+'imit'='LIMIT'
(Other definitions for limit that I've seen before include "Curb" , "Restrict - term" , "Restrict - boundary" , "As far as something can go" , "Put restriction on" .)