First to go in, make a bloomer (7)
I believe the answer is:
mistake
'a bloomer' is the definition.
(thesaurus)
'first to go in make' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'go in' means one lot of letters goes inside another.
'ist' placed inside 'make' is 'MISTAKE'.
(Other definitions for mistake that I've seen before include "Wrong action or judgment" , "16 down" , "Oversight" , "Slip" , "Error" .)