Second fight perhaps upset more gentle children (7)
I believe the answer is:
rematch
'second fight perhaps' is the definition.
I can't judge whether this defines the answer.
'upset more gentle children' is the wordplay.
'upset' shows that the letters should be reversed in order.
'more gentle' becomes 'tamer' ('tame' can be similar in meaning to 'gentle').
'children' becomes 'ch' (genealogical abbreviation).
'tamer' reversed gives 'remat'.
'remat'+'ch'='REMATCH'
Can you help me to learn more?
(Other definitions for rematch that I've seen before include "Later encounter" , "Teams' second game" , "new bout" , "Return fixture" , "Further engagement of the same contestants" .)