Hold on - 151's no good! (5)
I believe the answer is:
cling
'hold on' is the definition.
(I've seen this in another clue)
'151's no good' is the wordplay.
'151' becomes 'CLI' (Roman numerals).
'no good' becomes 'ng' (abbreviation for no and good).
'cli'+'ng'='CLING'
(Other definitions for cling that I've seen before include "Grasp urgently" , "Stick - tinkle" , "act like limpets" , "Hold, cleave" , "Hold on as for dear life" .)