Best pick by interim sides joining A-League (7)
I believe the answer is:
optimal
'best' is the definition.
(optimal means the best)
'pick by interim sides joining a-league' is the wordplay.
'pick by' becomes 'opt' (as in opting for or picking something. I am not sure about the 'by' bit.).
'sides' says to hollow out the word (remove centre letters) (only the letters on the sides are used).
'joining' is a charade indicator (letters next to each other).
'league' becomes 'l' (abbreviation).
'interim' with its middle taken out is 'im'.
'opt'+'im'+'a'+'l'='OPTIMAL'
Can you help me to learn more?
(Other definitions for optimal that I've seen before include "as good as can be" , "Plot aim (anag) - most favourable" , "Best or most favourable" , "Most favouring" , "Most desirable" .)