Trendy monarch conceals large clue (7)
I believe the answer is:
inkling
'clue' is the definition.
(I've seen this before)
'trendy monarch conceals large' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'monarch' becomes 'king' (king is a kind of monarch).
'conceals' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'king' going around 'l' is 'kling'.
'in'+'kling'='INKLING'
(Other definitions for inkling that I've seen before include "Suspicion, hunch" , "Intimation" , "Vague suspicion" , "Clue" , "Glimmering of notion" .)