Cardinal showing gravity — not weak, even at the end (8)
I believe the answer is:
eighteen
'cardinal' is the definition.
(similar in meaning)
'gravity not weak even at the end' is the wordplay.
'gravity' becomes 'weight' (both can mean importance or significance).
'not' is a deletion indicator.
'weak' becomes 'w' (physics symbol as in W-boson).
'even' becomes 'een' (old-fashioned contraction).
'at the end' is a charade indicator (letters next to each other).
'weight' with 'w' taken out is 'eight'.
'eight'+'een'='EIGHTEEN'
'showing' acts as a link.
(Other definitions for eighteen that I've seen before include "Treble six" , "age of maturity" , "The genie (anag.)" , "Old enough to vote" , "Greens on a golf course?" .)