Instruction when a group playing's trendy (7,4)
I believe the answer is:
abandon ship
'instruction' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'a group playing's trendy' is the wordplay.
'group' becomes 'band' (band can mean a group of people).
'playing's' becomes 'on's' (on can mean playing or performing).
'trendy' becomes 'hip' (I've seen this in another clue).
'a'+'band'+'ons'+'hip'='ABANDON SHIP'
'when' is the link.
Can you help me to learn more?
(Other definitions for abandon ship that I've seen before include "Go overboard" , "Leave a sinking vessel" , "get into lifeboats" .)