Clarebrough is distinguished for his contributions to physical metallurgy and metal physics. In particular he has studied atomic sized defects in metals and their relationship with macroscopic properties such as strength and plasticity. He developed high precision differential calorimetry for defect studies in many areas including a classical analysis of order-disorder phenomena in brass. More recently he has been in the forefront of the direct observation of defects by electron microscopy. The rigorous analysis of such observations, based on his contributions, is now in international usage.