So terribly near to catching and that’s why (6)
I believe the answer is:
reason
'that's why' is the definition.
(I've seen this in another clue)
'so terribly near to catching' is the wordplay.
'terribly' indicates an anagram.
'to catching' means one lot of letters goes inside another.
'near' is an anagram of 'rean'.
'so' inserted into 'rean' is 'REASON'.
'and' acts as a link.
(Other definitions for reason that I've seen before include "'Motivation, grounds (6)'" , "Deduce" , "Defence" , "Cause, justification" , "Cause or motive" .)