Make certain changes to username, though not in the morning (6)
I believe the answer is:
ensure
'make certain' is the definition.
(synonyms)
'changes to username though not in the morning' is the wordplay.
'changes to' indicates an anagram (I've seen 'change to' mean this).
'though not' suggests deleting specific letters.
'in the morning' becomes 'am' (AM on a clock).
'username' with 'am' taken away is 'userne'.
'userne' anagrammed gives 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain that (something occurs)" , "warrant" , "Make certain, clinch" , "Make certain (something will occur)" , "Make certain, confirm" .)