Writer in trainers popping pill (13)
I believe the answer is:
tranquilliser
'pill' is the definition.
'writer in trainers popping' is the wordplay.
'writer' becomes 'quill' (I have seen 'Old writer' mean 'quill' so perhaps 'writer' could also mean 'quill').
'in' is an insertion indicator.
'popping' indicates an anagram (the original order pops or explodes).
'trainers' is an anagram of 'traniser'.
'quill' put within 'traniser' is 'TRANQUILLISER'.
(Other definitions for tranquilliser that I've seen before include "Sedative" , "A number" , "Pill used to calm" , "Calming-down drug" , "Calming drug" .)