151 is briefly the upper limit, the culmination (6)
I believe the answer is:
climax
'culmination' is the definition.
(I know that culmination can be written as climax)
'151 is briefly the upper limit' is the wordplay.
I cannot really see how this works, but
'151' could be 'cli' (Roman numerals) and 'cli' is found within the answer.
'limit' could be 'max' (I've seen this in another clue) and 'max' is present in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for climax that I've seen before include "Turning point or culmination" , "(Reach the) highest level" , "The decisive moment, in a novel or play say" , "High point" , "Exciting conclusion" .)