Calls for reform of prison after start of debate (5,2)
I believe the answer is:
drops in
'calls' is the definition.
(dropping in is a kind of calling)
'reform of prison after start of debate' is the wordplay.
'reform of' is an anagram indicator (I've seen this in other clues).
'after' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The initial letter of 'debate' is 'd'.
'prison' anagrammed gives 'ropsin'.
'ropsin' after 'd' is 'DROPS IN'.
'for' acts as a link.
(Other definitions for drops in that I've seen before include "Comes to see" , "eye thus getting less inflamed?" , "Casually visits" , "Calls casually" .)