Key cricket club adopting different sport starts to inspire all league (7)
I believe the answer is:
crucial
'key' is the definition.
(thesaurus)
'cricket club adopting different sport starts to inspire all league' is the wordplay.
'cricket club' becomes 'cc'.
'adopting' is an insertion indicator.
'different sport' becomes 'ru' (rugby union. I am not sure about the 'different' bit.**).
'starts to' suggests taking the first letters.
The initial letters of 'inspire all league' is 'ial'.
'cc' placed around 'ru' is 'cruc'.
'cruc'+'ial'='CRUCIAL'
(Other definitions for crucial that I've seen before include "'Vital, essential (7)'" , "Of great importance" , "'Decisive, critical (7)'" , "Of essential importance" , "Of extreme importance" .)