Unfortunately, spare time gradually diminishes (6)
I believe the answer is:
tapers
'gradually diminishes' is the definition.
'tapers' can be an answer for 'diminishes' (tapering is a kind of diminishing). I am not certain of the 'gradually' bit.
'unfortunately spare time' is the wordplay.
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'time' becomes 't' (abbreviation).
'spare'+'t'='sparet'
'sparet' with letters rearranged gives 'TAPERS'.
Can you help me to learn more?
(Other definitions for tapers that I've seen before include "Narrows towards the end, like a candle" , "Repast with slender candles" , "Makes or becomes gradually narrower" , "Becomes gradually narrower as candle may" , "Diminishes gradually" .)