Grasp opening mechanism round nailhead (5,2)
I believe the answer is:
latch on
'grasp' is the definition.
(I know that grasp is a is a more specific form of the action of latch on)
'opening mechanism round nailhead' is the wordplay.
'opening mechanism' becomes 'latch' (I can't justify this - if you can you should give a lot more credence to this answer).
'round' becomes 'o' (round can mean a round object).
'nailhead' becomes 'n' (head letter of 'nail').
'latch'+'o'+'n'='LATCH ON'
Can you help me to learn more?
(Other definitions for latch on that I've seen before include "Understand" , "door is locked" .)