Clue: Hurry up and get big diaper on son (4,2,6)
Answer: MAKE IT SNAPPY
Explanation: Definition: Hurry up
 [MAKE IT](to get/to become big/successful and famous) + [ NAPPY(diaper/napkin of soft absorbent material to absorb a baby's excrement) **placed after**(on) S(abbrev. for "son") ].