Integral part of one's best suit (5,4)
I believe the answer is:
trump card
'integral part of one's best suit' is the definition.
I can't judge whether this definition defines the answer.
'best suit' is the wordplay.
'best' becomes 'trump' (synonyms).
'suit' becomes 'card' (I can't explain this - if you can you should believe this answer much more).
'trump'+'card'='TRUMP-CARD'
Can you help me to learn more?
(Other definitions for trump card that I've seen before include "Key resource to be used at the right moment" , "Useful expedient" , "Trick winner" , "Secret advantage" , "winning move" .)