O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
To “hold” here probably means “restrain,” although visually the youth can also seem to be handling “Time’s” “glass” and “hour.”   “glass” may be a  looking-glass  or mirror, which would reflect the process of a person’s ageing–or, it could be an  hour-glass . 
 The use of “sickle” to describe the hour calls to mind both a waning moon (see line 3) and the reaper’s scythe. 
 Note also the internally rhyming “fickle” and “sickle”. The hard, percussive consonants — the “ck"s — are appropriate in relation to the subject, yet the lines are rhythmic and poignant. 
 
 Jean de la Fontaine’s  Death and the Woodcutter.
Sonnet 126