Write entering very large competitions (5)
I believe the answer is:
opens
'competitions' is the definition.
(open is a kind of competition)
'write entering very large' is the wordplay.
'write' becomes 'pen' (synonyms).
'entering' means one lot of letters goes inside another.
'very large' becomes 'os' (abbreviation for oversize).
'pen' inserted into 'os' is 'OPENS'.
(Other definitions for opens that I've seen before include "releases" , "Exposes, begins" , "sports competitions" , "Unfastens" , "Launches" .)