Scotsman in good time finds mammoth (5)
I believe the answer is:
giant
'mammoth' is the definition.
(I've seen this in another clue)
'scotsman in good time' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'in' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'time' becomes 't' (abbreviation).
'g'+'t'='gt'
'ian' inserted within 'gt' is 'GIANT'.
'finds' is the link.
(Other definitions for giant that I've seen before include "Huge; James Dean film" , "Ogre" , "Monster" , "Very big person" , "Colossus" .)