Make strong enough potty to support tot's bottom (7)
I believe the answer is:
toughen
'make strong' is the definition.
(I've seen this before)
'enough potty to support tot's bottom' is the wordplay.
'potty' indicates an anagram.
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'bottom' suggests the final letters.
The last letter of 'tot' is 't'.
'enough' with letters rearranged gives 'oughen'.
'oughen' put after 't' is 'TOUGHEN'.
(Other definitions for toughen that I've seen before include "Increase stiffness" , "Strengthen, season" , "Make more robust" , "Make more hardy or durable" , "Make strong and durable" .)