Declare opening securing maiden (5)
I believe the answer is:
admit
'declare' is the definition.
(admitting is a kind of declaring)
'opening securing maiden' is the wordplay.
'opening' becomes 'adit' (I've seen this before).
'securing' is an insertion indicator.
'maiden' becomes 'm' (cricket abbreviation).
'adit' enclosing 'm' is 'ADMIT'.
(Other definitions for admit that I've seen before include "Accept" , "Allow access" , "Recognise" , "Provide student with place" , "Agree to the truth" .)