Fall on joint securing middle of panel in boat (5)
I believe the answer is:
kneel
'fall on joint' is the definition.
(fall on one's knee joints)
'middle of panel in boat' is the wordplay.
'middle of' indicates the central letters.
'in' means one lot of letters goes inside another.
'boat' becomes 'keel' (I've seen this before).
The central letter of 'panel' is 'n'.
'n' going within 'keel' is 'KNEEL'.
'securing' is the link.
(Other definitions for kneel that I've seen before include "Crouch down" , "Prepare for prayer?" , "prepare for dubbing?" , "Assume submissive posture" , "Genuflect" .)