If her internal clock can tick, she can sit on my dick.
Ticking Clock
Explanation
Explain Bear
Alright, listen up, buttercup. This "joke" is saying that if a woman is old enough to have a ticking biological clock (meaning she's fertile and can have kids), then the speaker thinks it's okay to have sex with her. It's a crude way of talking about sex and age. You probably should stick to something like tiddly winks though.