When he's on strike, his partner's a comparative onlooker (7)
I believe the answer is:
batsman
'strike his partner's a comparative onlooker' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'when he's' is the wordplay.
'when' becomes 'bats' (I can't justify this - if you can you should give a lot more credence to this answer).
'he' becomes 'man' (I've seen this in another clue).
'bats'+'man'='BATSMAN'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for batsman that I've seen before include "Cricketer at the crease" , "Player at crease" , "Grace, for example" , "one of two on the field" , "He's either in or he's out!" .)