King seen, perhaps, giving party (5,2)
I believe the answer is:
knees up
'party' is the definition.
(I have seen 'Lively party' mean 'knees up' so perhaps 'party' could also mean 'knees up')
'king seen perhaps giving' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'perhaps' indicates anagramming the letters.
'giving' becomes 'up' (I am not sure about this - if you are sure you should believe this answer much more).
'seen' is an anagram of 'nees'.
'k'+'nees'+'up'='KNEES-UP'
(Other definitions for knees up that I've seen before include "party swing?" , "Lively party" .)