Hold on tightly (5)
I believe the answer is:
cling
'hold on tightly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cling that I've seen before include "Adhere to - ring" , "Hold, cleave" , "Grasp urgently" , "act like limpets" , "Stick - tinkle" .)