Weakest of the family or race to have rat's tail (4)
I believe the answer is:
runt
'weakest of the family' is the definition.
I can't tell whether this definition defines the answer.
'race to have rat's tail' is the wordplay.
'race' becomes 'run' (synonyms).
'to have' means one lot of letters go next to another.
'tail' suggests the final letters.
The final letter of 'rat' is 't'.
'run'+'t'='RUNT'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for runt that I've seen before include "Smallest piglet of a litter" , "Weak little squealer" , "Turn into smallest of the litter" , "Smallest animal in a litter" , "Littlest oinker" .)