Depth? Start to relish one board game involving knight (8)

'depth?' is the definition.
(thesaurus)

'start to relish one board game involving knight' is the wordplay.
'start to' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'board game' becomes 'chess' (chess is a kind of board game).
'involving' is an insertion indicator.
'knight' becomes 'N' (chess abbreviation).
The initial letter of 'relish' is 'r'.
'chess' going around 'n' is 'chness'.
'r'+'i'+'chness'='RICHNESS'