Royal starts to moan about libel after article produced by Heat (7)
I believe the answer is:
thermal
'produced by heat' is the definition.
(relating to or produced by heat)
'royal starts to moan about libel after article' is the wordplay.
'royal' becomes 'r' (abbreviation).
'starts to' indicates taking the first letters.
'about libel' becomes 'al' (this could be a standard abbreviation of which I'm unaware).
'after' says to put letters next to each other.
'article' becomes 'the' (the definite article in English).
The first letter of 'moan' is 'm'.
'r'+'m'+'al'='rmal'
'rmal' after 'the' is 'THERMAL'.
(Other definitions for thermal that I've seen before include "Pertaining to heat" , "Warm or hot - underwear?" , "A lot of hot air" , "Related to heat" , "Updraught for gliders" .)