Ring infiltrated by agent finally, a bit of a bloomer (5)
I believe the answer is:
petal
'a bit of a bloomer' is the definition.
(part of a flower, something which blooms)
'ring infiltrated by agent finally' is the wordplay.
'ring' becomes 'peal' (synonyms).
'infiltrated by' means one lot of letters goes inside another.
'finally' says to take the final letters.
The final letter of 'agent' is 't'.
'peal' enclosing 't' is 'PETAL'.
(Other definitions for petal that I've seen before include "Part of flag" , "part of seven [VIOLA]" , "part taken from stock?" , "Floral segment" , "One piece of corolla" .)