Enlarge one's gunstock again (5)
I believe the answer is:
rearm
'enlarge' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'one's gunstock again' is the wordplay.
I cannot really see how this works, but
'again' could be 're' (prefix meaning 'again') and 're' is found within the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rearm that I've seen before include "Up the military budget?" , "Take weapons again" , "Resupply with weapons" , "Strengthen one's defences" , "Provide new weaponry" .)