Reduce with diet pill oddly taken every Tuesday evening to begin with (7)
I believe the answer is:
deplete
'reduce' is the definition.
(I've seen this before)
'diet pill oddly taken every tuesday evening to begin with' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'taken' says to put letters next to each other.
'to begin with' indicates taking the first letters.
The initial letters of 'every tuesday evening' is 'ete'.
'diet'+'pill'='dietpill'
The alternate letters of 'dietpill' are 'depl'.
'depl'+'ete'='DEPLETE'
'with' is the link.
(Other definitions for deplete that I've seen before include "Use up, exhaust" , "Reduce (amount of supplies etc)" , "Drain" , "Run down (e.g. stocks)" , "Spend" .)