It's up to Mike, little man (3)
I believe the answer is:
tim
'little man' is the definition.
'Tim' can be an answer for 'man' (short for Timothy). I am unsure of the 'little' bit.
'it's up to mike' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'to mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'it' reversed gives 'ti'.
'ti'+'m'='TIM'
Can you help me to learn more?
(Other definitions for tim that I've seen before include "Tiny Dickens boy from 'A Christmas Carol'" , "-- Brooke-Taylor, comedian; -- Minchin, comedian" , "Sir -- Rice, lyricist" , "Tiny ..., Bob Cratchit's son in A Christmas Carol" , "Little fellow" .)