Overthrow permitted for two (7)
I believe the answer is:
couplet
'two' is the definition.
(couplet is a kind of two)
'overthrow permitted' is the wordplay.
'overthrow' becomes 'coup' (I've seen this in another clue).
'permitted' becomes 'let' (to let something happen is to permit it).
'coup'+'let'='COUPLET'
'for' acts as a link.
(Other definitions for couplet that I've seen before include "bit of poetry" , "Two successive lines of verse, usually rhyming" , "Two lines of verse, often rhyming" , "Pair of lines in poetry" , "piece of poetry" .)