Seal that’s stoutest left female (6)
I believe the answer is:
attest
'seal' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Give evidence - sounds a trial" , "Give sworn evidence" , "Vouch for" , "Demonstrate" , "Declare (that something is the case)" .)