Frenchman's tart securing boob (6)
I believe the answer is:
pierre
'frenchman's' is the definition.
(I've seen this before)
'tart securing boob' is the wordplay.
'tart' becomes 'pie' (I've seen this before).
'securing' indicates putting letters inside.
'boob' becomes 'err' (I've seen this before).
'pie' enclosing 'err' is 'PIERRE'.
(Other definitions for pierre that I've seen before include "Peter in France" , "French boy's name" , "See 17" , "Frenchman" , "Peter, to the French" .)