If in a certain code 'DAUGHTER' is written as 'TERDAUGH' , how will 'APTITUDE' be written in that code?

Split the word into two parts and append the first part at the end of second.

From question it is clear that we need to make 2 words from word APTITUDE where first word should have first 5 alphabets and remaining 3 alphabets will be part of 2nd word.

APTITUDE ? APTIT + UDE ? UDEAPTIT

So the code for APTITUDE is UDEAPTIT.