Tight lip involved, about love, principally old flame (5,5)
I believe the answer is:
pilot light
'flame' is the definition.
(I've seen this before)
'tight lip involved about love principally old' is the wordplay.
'involved' is an anagram indicator (involved can mean confusing or complex).
'about' means one lot of letters goes inside another.
'principally' suggests taking the first letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The first letter of 'love' is 'l'.
'tight'+'lip'='tightlip'
'tightlip' anagrammed gives 'pitlight'.
'l'+'o'='lo'
'pitlight' placed around 'lo' is 'PILOT LIGHT'.
(Other definitions for pilot light that I've seen before include "It inflames" , "Airline slightly short-staffed?" , "It must flaming well keep going" , "ignition system" , "Indicator" .)