If in no end of doubt before close of play, declare! (6)
I believe the answer is:
notify
'declare' is the definition.
(thesaurus)
'if in no end of doubt before close of play' is the wordplay.
'in' indicates putting letters inside.
'end of' suggests the final letters.
'before close of play' becomes 'y' (I've seen this in other clues).
The last letter of 'doubt' is 't'.
'no'+'t'+'y'='noty'
'if' going inside 'noty' is 'NOTIFY'.
(Other definitions for notify that I've seen before include "Formally inform" , "Officially inform" , "Declare" , "Make known" , "alert" .)