It covers physical damage caused by small carrier (4)
I believe the answer is:
scab
'it covers physical damage' is the definition.
'small carrier' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'carrier' becomes 'cab' (**).
's'+'cab'='SCAB'
'caused by' is the link.
(Other definitions for scab that I've seen before include "evidence of recent injury" , "Scoundrel" , "cause of itch?" , "complaint with scales" , "part of healing process" .)