Three left
10a Whole number,say, in between second groove: other things are beastly (7,6)
The 7 answer is integer, which is the whole number so all or part of the rest of the clue must refer to the 6 answer _R_T_L
25a Greek god and Persian king I leave to Trojan, who went between a lion and a 5, whence its useless to this. (8:2,5)
Now the 2,5 answer is no avail and the letters I have for the 8 answer are _A_D_R_S
4d Tiny creature, an island setter, entertaining somewhat calculating Teutonic pacemaker (10:5) the 10 part being animalcula and the 5 has letters G_R_Y
Hope you can trawl through all these convoluted explanations - am addled at end of this crossword!!!
10a in between belongs to the first part, say =eg in "between" inter, hence integer.
Second answer is brutal - second = b, groove = rut, other things = al.
25a Pandarus (Greek god = Pan, Persian king = Darius without i)