Starts to write, in big letters (5)
I believe the answer is:
opens
'starts' is the definition.
(opening is a kind of starting)
'write in big letters' is the wordplay.
I cannot quite see how this works, but
'write' could be 'pen' (synonyms) and 'pen' is found within the answer.
'big' could be 'os' and 'os' is located in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for opens that I've seen before include "releases" , "Tournaments for both professionals and amateurs" , "Launches" , "Removes a lid, say" , "Establishes" .)