Drop trousers I had split (6)
I believe the answer is:
divide
'split' is the definition.
(synonyms)
'drop trousers i had' is the wordplay.
'drop' becomes 'dive' (I am not sure about this - if you are sure you should believe this answer much more).
'trousers' indicates putting letters inside (to trouser can mean to steal or take).
'i had' becomes 'I'd' (common contraction).
'dive' enclosing 'id' is 'DIVIDE'.
Can you help me to learn more?
(Other definitions for divide that I've seen before include "draw between 13A & 19" , "Split in two" , "Saying : '. . . . . . and conquer'" , "Split into portions" , "Partition. separate" .)