Be unhappy about the dilapidated pier, by the way (6)
I believe the answer is:
repine
'be' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.
'unhappy about the dilapidated pier by the way' is the wordplay.
'unhappy' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'the dilapidated' is an anagram indicator.
'by' means one lot of letters go next to another.
'the way' becomes 'e' (Compass direction - East).
'pier' put after 'e' is 'epier'.
'epier' with letters rearranged gives 'repie'.
'n' inserted into 'repie' is 'REPINE'.
Can you help me to learn more?
(Other definitions for repine that I've seen before include "Fret, about a tree perhaps?" , "Be in a state of anxiety" , "Fret, be discontented" , "Express discontent, fret" , "Plain" .)