Will antibiotics (clamentin) and cetirizine help to clear fluid in the ears ( serous otitis media ) ?

Not necessarily: Retention of middle ear fluid is more a function of poor Eustachian tube drainage than anything else. Trapping fluid could allow germs to have food/water to live on & killing them may help. If none are present it does nothing. Antihistamines can thicken the fluid & lessen chance it would drain. You can just try & see. It may/not work.

Harm > benefit: The fluid usually resolves on its own within 4-6 weeks, and antibiotics in particular may cause more harm (creating resistant bacteria) than help. However, if it lasts months or if you significant, acute pain or fever of 39◦C / 102◦F, you may want to start the medications with a doctor’s input.