One’s not quite a cheat - sneakily manages to secure First in maths (8)

'one's not quite a cheat' is the definition.
(a gamesman's tactics aren't technically cheating)

'sneakily manages to secure first in maths' is the wordplay.
'sneakily' indicates an anagram (a tricksy or sneaky ordering of the letters).
'to secure' indicates putting letters inside.
'first in maths' becomes 'm' (1st letter of 'maths').
'manages' with letters rearranged gives 'gamesan'.
'gamesan' placed around 'm' is 'GAMESMAN'.