Sincerity of fellow crossing a loch (11)
I believe the answer is:
earnestness
'sincerity' is the definition.
(I know that sincerity can be written as earnestness)
'fellow crossing a loch' is the wordplay.
'fellow' becomes 'ernest' (I've seen this before).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'loch' becomes 'ness' (Loch Ness in Scotland).
'ernest' enclosing 'a' is 'earnest'.
'earnest'+'ness'='EARNESTNESS'
'of' acts as a link.
(Other definitions for earnestness that I've seen before include "Intense seriousness" , "Sincerity" .)