Grasp chain running under boat (5,2)
I believe the answer is:
catch on
'grasp' is the definition.
(both can mean to realise something)
'chain running under boat' is the wordplay.
I cannot quite understand how this works, but
'chain' could be 'ch' (I've seen this in other clues) and 'ch' is present in the answer.
'running' could be 'on' (eg when a machine is running it is on) and 'on' is located in the answer.
'boat' could be 'cat' (short for catboat) and 'cat' is present in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for catch on that I've seen before include "Informally, become popular or come to understand" , "twig" , "Understand - become fashionable" , "Get it" .)