Keep rare pair of diamonds, including queen, in card game (7)
I believe the answer is:
underdo
'queen in card game' is the definition.
The definition suggests a verb in its base form which matches the answer.
'keep rare pair of diamonds' is the wordplay.
I cannot quite see how this works, but
'rare' could be 'r' (listed by Oxford Dictionary of Abbreviations) and 'r' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
'diamonds' could be 'd' (abbreviation for the card suit diamonds) and 'd' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'including' acts as a link.
Can you help me to learn more?
(Other definitions for underdo that I've seen before include "Cook in a bad way" , "Cook for short time" , "Cook too briefly" , "Rarely cook" , "Be responsible for rare product" .)