Hungry royal perhaps ate sides up (5)
I believe the answer is:
eager
'hungry' is the definition.
(I've seen this before)
'royal perhaps ate sides up' is the wordplay.
'royal' becomes 'r' (abbreviation).
'perhaps' becomes 'eg' (for example).
'sides' means to remove the middle letters (only the letters on the sides are used).
'up' is a reversal indicator (in down clue: letters go upwards).
'ate' with its centre removed is 'ae'.
'r'+'eg'+'ae'='regae'
'regae' backwards is 'EAGER'.
(Other definitions for eager that I've seen before include "Alert and active type?" , "Excited by desire" , "Anxious" , "Keen - enthusiastic" , "Agree (anag.)" .)