Puzzle of extremely reliable boat capsizing (5)
I believe the answer is:
rebus
'puzzle' is the definition.
(type of visual puzzle)
'extremely reliable boat capsizing' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'boat' becomes 'sub' (submarine).
'capsizing' is a reversal indicator.
'reliable' with its middle removed is 're'.
'sub' back-to-front is 'bus'.
're'+'bus'='REBUS'
'of' acts as a link.
(Other definitions for rebus that I've seen before include "Word-picture puzzle" , "Puzzle consisting of pictures representing syllables and words" , "Pictorial teaser" , "Word puzzle involving pictures" , "Puzzling device" .)