Tenniel could be so merciful and forbearing (7)
I believe the answer is:
lenient
'forbearing' is the definition.
(I've seen this before)
'tenniel could be' is the wordplay.
'could be' is an anagram indicator.
'tenniel' with letters rearranged gives 'LENIENT'.
'so merciful and' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lenient that I've seen before include "Forgiving, soft" , "'Indulgent, easy (7)'" , "Not severe, merciful" , "Mild and tolerant" , "Clement" .)