Having to stand up to him (4)
I believe the answer is:
otto
'him' is the definition.
'having to stand up to' is the wordplay.
'having' becomes 'to' (I am not sure about this - if you are sure you should believe this answer much more).
'to stand up' is a reversal indicator (in a down clue, the word stands upside down).
'to' written backwards gives 'ot'.
'ot'+'to'='OTTO'
Can you help me to learn more?
(Other definitions for otto that I've seen before include "Rose produces this" , "Ger. name (palindrome)" , "____ Preminger, US film director born in Austria" , "Preminger or von Bismark?" , "German man's name" .)