Place of seclusion used by engineers before pleasant occasion (7)
I believe the answer is:
retreat
'place of seclusion' is the definition.
'retreat' can be an answer for 'seclusion' (thesaurus). I'm unsure of the 'place of' bit.
'by engineers before pleasant occasion' is the wordplay.
'by' is a charade indicator (letters next to each other).
'engineers' becomes 're' (Royal Engineers).
'before pleasant occasion' becomes 'treat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
're' next to 'treat' is 'RETREAT'.
'used' acts as a link.
Can you help me to learn more?
(Other definitions for retreat that I've seen before include "retirement" , "unhappy manoeuvre" , "concession" , "asylum" , "Back away" .)