Adding width to random lagoon is a blunder (3,4)
I believe the answer is:
own goal
'a blunder' is the definition.
'adding width to random lagoon' is the wordplay.
I cannot quite see how this works, but
'width' could be 'w' (abbreviation in geometry) and 'w' is found in the answer.
an anagram of 'lagoon' is 'ongoal' which is within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for own goal that I've seen before include "or occasionally with it?" , "action that backfired?" , "Personal ambition" , "Costly footballing error" , "Self-inflicted wound" .)