Showing serious conviction, moves fourth letter to the top so that it will be the closest (7)
I believe the answer is:
nearest
'the closest' is the definition.
(I've seen this before)
'showing serious conviction moves fourth letter to the top so that it will be' is the wordplay.
I cannot quite see how this works, but
'letter' could be 't' and 't' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
'be' could be 'are' (I've seen this in another clue) and 'are' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nearest that I've seen before include "Closest, being earnest" , "In greatest propinquity" , "Most approximate" , "Most local" , "most like Scrooge at first" .)