Think of a number and write it down (7)
I believe the answer is:
compose
'down' is the definition.
(thesaurus)
'think of a number and write it' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is located in the answer.
'number' could be 'm' (M is an example) and 'm' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for compose that I've seen before include "What Ireland used to do" , "Some cop (anag)" , "Write a piece of music" , "Make up, create" , "Create or put together" .)