Ampere truly altered before time (11)
I believe the answer is:
prematurely
'before time' is the definition.
(I've seen this before)
'ampere truly altered' is the wordplay.
'altered' is an anagram indicator.
'ampere'+'truly'='amperetruly'
'amperetruly' with letters rearranged gives 'PREMATURELY'.
(Other definitions for prematurely that I've seen before include "before it's ready?" , "Before the due time" .)